diff --git a/integrationtest/command/testdata/gitlab_output/stdout/stdout.txt b/integrationtest/command/testdata/gitlab_output/stdout/stdout.txt
index d675074..1fba7d1 100644
--- a/integrationtest/command/testdata/gitlab_output/stdout/stdout.txt
+++ b/integrationtest/command/testdata/gitlab_output/stdout/stdout.txt
@@ -5,7 +5,11 @@
     "fingerprint": "e9b14e45ca01a9a72fda9b8356a9ddbbaf7fe8c47116790a51cd699ae1679353",
     "severity": "major",
     "location": {
-      "path": "needs_format.yaml"
+      "path": "needs_format.yaml",
+      "lines": {
+        "begin": 2,
+        "end": 4
+      }
     }
   }
 ]
diff --git a/internal/gitlab/codequality.go b/internal/gitlab/codequality.go
index 6643322..28492fd 100644
--- a/internal/gitlab/codequality.go
+++ b/internal/gitlab/codequality.go
@@ -18,6 +18,7 @@ package gitlab
 import (
 	"crypto/sha256"
 	"fmt"
+	"strings"
 
 	"github.com/google/yamlfmt"
 )
@@ -35,7 +36,14 @@ type CodeQuality struct {
 
 // Location is the location of a Code Quality finding.
 type Location struct {
-	Path string `json:"path,omitempty"`
+	Path  string `json:"path,omitempty"`
+	Lines *Lines `json:"lines,omitempty"`
+}
+
+// Lines follows the GitLab Code Quality schema.
+type Lines struct {
+	Begin int  `json:"begin"`
+	End   *int `json:"end,omitempty"`
 }
 
 // NewCodeQuality creates a new CodeQuality object from a yamlfmt.FileDiff.
@@ -46,6 +54,8 @@ func NewCodeQuality(diff yamlfmt.FileDiff) (CodeQuality, bool) {
 		return CodeQuality{}, false
 	}
 
+	begin, end := detectChangedLines(&diff)
+
 	return CodeQuality{
 		Description: "Not formatted correctly, run yamlfmt to resolve.",
 		Name:        "yamlfmt",
@@ -53,10 +63,51 @@ func NewCodeQuality(diff yamlfmt.FileDiff) (CodeQuality, bool) {
 		Severity:    Major,
 		Location: Location{
 			Path: diff.Path,
+			Lines: &Lines{
+				Begin: begin,
+				End:   &end,
+			},
 		},
 	}, true
 }
 
+// detectChangedLines finds the first and last lines that differ between original and formatted content.
+func detectChangedLines(diff *yamlfmt.FileDiff) (begin, end int) {
+	original := strings.Split(string(diff.Diff.Original), "\n")
+	formatted := strings.Split(string(diff.Diff.Formatted), "\n")
+
+	maxLines := max(len(original), len(formatted))
+
+	begin = -1
+	end = -1
+
+	for i := 0; i < maxLines; i++ {
+		origLine := ""
+		fmtLine := ""
+
+		if i < len(original) {
+			origLine = original[i]
+		}
+		if i < len(formatted) {
+			fmtLine = formatted[i]
+		}
+
+		if origLine != fmtLine {
+			if begin == -1 {
+				begin = i + 1
+			}
+			end = i + 1
+		}
+	}
+
+	if begin == -1 {
+		begin = 1
+		end = 1
+	}
+
+	return begin, end
+}
+
 // fingerprint returns a 256-bit SHA256 hash of the original unformatted file.
 // This is used to uniquely identify a code quality finding.
 func fingerprint(diff yamlfmt.FileDiff) string {
diff --git a/internal/gitlab/codequality_test.go b/internal/gitlab/codequality_test.go
index 52c8a1c..6a02d96 100644
--- a/internal/gitlab/codequality_test.go
+++ b/internal/gitlab/codequality_test.go
@@ -16,10 +16,13 @@ package gitlab_test
 
 import (
 	"encoding/json"
+	"os"
+	"path/filepath"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/yamlfmt"
+	"github.com/google/yamlfmt/internal/assert"
 	"github.com/google/yamlfmt/internal/gitlab"
 )
 
@@ -66,30 +69,256 @@ func TestCodeQuality(t *testing.T) {
 			t.Parallel()
 
 			got, gotOK := gitlab.NewCodeQuality(tc.diff)
-			if gotOK != tc.wantOK {
-				t.Fatalf("NewCodeQuality() = (%#v, %v), want (*, %v)", got, gotOK, tc.wantOK)
-			}
+			assert.Equal(t, tc.wantOK, gotOK)
 			if !gotOK {
 				return
 			}
 
-			if tc.wantFingerprint != "" && tc.wantFingerprint != got.Fingerprint {
-				t.Fatalf("NewCodeQuality().Fingerprint = %q, want %q", got.Fingerprint, tc.wantFingerprint)
+			if tc.wantFingerprint != "" {
+				assert.Equal(t, tc.wantFingerprint, got.Fingerprint)
 			}
 
 			data, err := json.Marshal(got)
-			if err != nil {
-				t.Fatal(err)
-			}
+			assert.NilErr(t, err)
 
 			var gotUnmarshal gitlab.CodeQuality
-			if err := json.Unmarshal(data, &gotUnmarshal); err != nil {
-				t.Fatal(err)
+			err = json.Unmarshal(data, &gotUnmarshal)
+			assert.NilErr(t, err)
+
+			if d := cmp.Diff(got, gotUnmarshal); d != "" {
+				assert.EqualMsg(t, "", d, "json.Marshal() and json.Unmarshal() mismatch (-got +want):\n%s")
 			}
+		})
+	}
+}
+
+func TestCodeQuality_DetectChangedLine(t *testing.T) {
+	t.Parallel()
+
+	testdataDir := "./testdata/changed_line"
+	originalPath := filepath.Join(testdataDir, "original.yaml")
+	formattedPath := filepath.Join(testdataDir, "formatted.yaml")
+
+	original, err := os.ReadFile(originalPath)
+	assert.NilErr(t, err)
+
+	formatted, err := os.ReadFile(formattedPath)
+	assert.NilErr(t, err)
+
+	diff := yamlfmt.FileDiff{
+		Path: "testdata/original.yaml",
+		Diff: &yamlfmt.FormatDiff{
+			Original:  original,
+			Formatted: formatted,
+		},
+	}
+
+	cq, ok := gitlab.NewCodeQuality(diff)
+	assert.Assert(t, ok, "NewCodeQuality() returned false, expected true")
+
+	assert.Assert(t, cq.Location.Lines != nil, "Location.Lines is nil")
+
+	wantBeginLine := 6
+	gotBeginLine := cq.Location.Lines.Begin
+	assert.Equal(t, wantBeginLine, gotBeginLine)
+
+	assert.Assert(t, cq.Location.Lines.End != nil, "Location.Lines.End is nil")
+
+	wantEndLine := 8
+	gotEndLine := *cq.Location.Lines.End
+	assert.Equal(t, wantEndLine, gotEndLine)
+
+	assert.Equal(t, diff.Path, cq.Location.Path)
+
+	assert.Assert(t, cq.Description != "", "Description is empty")
+	assert.Assert(t, cq.Name != "", "Name is empty")
+	assert.Assert(t, cq.Fingerprint != "", "Fingerprint is empty")
+	assert.Assert(t, cq.Severity != "", "Severity is empty")
+}
+
+func TestCodeQuality_DetectChangedLines_FromTestdata(t *testing.T) {
+	t.Parallel()
+
+	type tc struct {
+		name      string
+		dir       string
+		wantOK    bool
+		wantBegin int
+		wantEnd   int
+	}
+
+	cases := []tc{
+		{
+			name:   "no lines changed",
+			dir:    "no_lines_changed",
+			wantOK: false,
+		},
+		{
+			name:      "all lines changed",
+			dir:       "all_lines_changed",
+			wantOK:    true,
+			wantBegin: 1,
+			wantEnd:   2,
+		},
+		{
+			name:      "single line changed",
+			dir:       "single_line_changed",
+			wantOK:    true,
+			wantBegin: 2,
+			wantEnd:   2,
+		},
+		{
+			name:      "only the last line changed",
+			dir:       "last_line_changed",
+			wantOK:    true,
+			wantBegin: 3,
+			wantEnd:   3,
+		},
+		{
+			name:      "only change is appending a last line",
+			dir:       "append_last_line",
+			wantOK:    true,
+			wantBegin: 3,
+			wantEnd:   3,
+		},
+	}
+
+	for _, c := range cases {
+		c := c
+
+		t.Run(c.name, func(t *testing.T) {
+			t.Parallel()
+
+			testdataDir := filepath.Join("./testdata/changed_line", c.dir)
+			originalPath := filepath.Join(testdataDir, "original.yaml")
+			formattedPath := filepath.Join(testdataDir, "formatted.yaml")
 
-			if diff := cmp.Diff(got, gotUnmarshal); diff != "" {
-				t.Errorf("json.Marshal() and json.Unmarshal() mismatch (-got +want):\n%s", diff)
+			original, err := os.ReadFile(originalPath)
+			assert.NilErr(t, err)
+
+			formatted, err := os.ReadFile(formattedPath)
+			assert.NilErr(t, err)
+
+			diff := yamlfmt.FileDiff{
+				Path: filepath.ToSlash(filepath.Join("testdata/changed_line", c.dir, "original.yaml")),
+				Diff: &yamlfmt.FormatDiff{
+					Original:  original,
+					Formatted: formatted,
+				},
+			}
+
+			cq, ok := gitlab.NewCodeQuality(diff)
+			assert.Equal(t, c.wantOK, ok)
+			if !ok {
+				return
+			}
+
+			assert.Assert(t, cq.Location.Lines != nil, "Location.Lines is nil")
+			assert.Equal(t, c.wantBegin, cq.Location.Lines.Begin)
+
+			assert.Assert(t, cq.Location.Lines.End != nil, "Location.Lines.End is nil")
+			assert.Equal(t, c.wantEnd, *cq.Location.Lines.End)
+
+			assert.Equal(t, diff.Path, cq.Location.Path)
+			assert.Assert(t, cq.Description != "", "Description is empty")
+			assert.Assert(t, cq.Name != "", "Name is empty")
+			assert.Assert(t, cq.Fingerprint != "", "Fingerprint is empty")
+			assert.Assert(t, cq.Severity != "", "Severity is empty")
+		})
+	}
+}
+
+func TestCodeQuality_DetectChangedLines_MultipleCases(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name      string
+		original  string
+		formatted string
+		wantBegin int
+		wantEnd   int
+	}{
+		{
+			name:      "single line change",
+			original:  "a:   b",
+			formatted: "a: b",
+			wantBegin: 1,
+			wantEnd:   1,
+		},
+		{
+			name: "multiple consecutive lines",
+			original: `line1
+line2:   value
+line3:  value
+line4`,
+			formatted: `line1
+line2: value
+line3: value
+line4`,
+			wantBegin: 2,
+			wantEnd:   3,
+		},
+		{
+			name: "non-consecutive changes",
+			original: `line1
+line2:   value
+line3
+line4:  value
+line5`,
+			formatted: `line1
+line2: value
+line3
+line4: value
+line5`,
+			wantBegin: 2,
+			wantEnd:   4,
+		},
+		{
+			name: "change at beginning",
+			original: `key:   value
+line2
+line3`,
+			formatted: `key: value
+line2
+line3`,
+			wantBegin: 1,
+			wantEnd:   1,
+		},
+		{
+			name: "change at end",
+			original: `line1
+line2
+key:   value`,
+			formatted: `line1
+line2
+key: value`,
+			wantBegin: 3,
+			wantEnd:   3,
+		},
+	}
+
+	for _, tc := range cases {
+		tc := tc
+
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			diff := yamlfmt.FileDiff{
+				Path: "test.yaml",
+				Diff: &yamlfmt.FormatDiff{
+					Original:  []byte(tc.original),
+					Formatted: []byte(tc.formatted),
+				},
 			}
+
+			cq, ok := gitlab.NewCodeQuality(diff)
+			assert.Assert(t, ok, "NewCodeQuality() returned false, expected true")
+
+			assert.Assert(t, cq.Location.Lines != nil, "Location.Lines is nil")
+			assert.Equal(t, tc.wantBegin, cq.Location.Lines.Begin)
+
+			assert.Assert(t, cq.Location.Lines.End != nil, "Location.Lines.End is nil")
+			assert.Equal(t, tc.wantEnd, *cq.Location.Lines.End)
 		})
 	}
 }
diff --git a/internal/gitlab/testdata/changed_line/all_lines_changed/formatted.yaml b/internal/gitlab/testdata/changed_line/all_lines_changed/formatted.yaml
new file mode 100644
index 0000000..2d0039a
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/all_lines_changed/formatted.yaml
@@ -0,0 +1,2 @@
+x: y
+z: w
diff --git a/internal/gitlab/testdata/changed_line/all_lines_changed/original.yaml b/internal/gitlab/testdata/changed_line/all_lines_changed/original.yaml
new file mode 100644
index 0000000..5851568
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/all_lines_changed/original.yaml
@@ -0,0 +1,2 @@
+a: b
+c: d
diff --git a/internal/gitlab/testdata/changed_line/append_last_line/formatted.yaml b/internal/gitlab/testdata/changed_line/append_last_line/formatted.yaml
new file mode 100644
index 0000000..efe5502
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/append_last_line/formatted.yaml
@@ -0,0 +1,3 @@
+line1: ok
+line2: ok
+line3: added
diff --git a/internal/gitlab/testdata/changed_line/append_last_line/original.yaml b/internal/gitlab/testdata/changed_line/append_last_line/original.yaml
new file mode 100644
index 0000000..cf9b973
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/append_last_line/original.yaml
@@ -0,0 +1,2 @@
+line1: ok
+line2: ok
diff --git a/internal/gitlab/testdata/changed_line/formatted.yaml b/internal/gitlab/testdata/changed_line/formatted.yaml
new file mode 100644
index 0000000..413d266
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/formatted.yaml
@@ -0,0 +1,18 @@
+# Example configuration file
+version: 1.0
+
+services:
+  web:
+    image: nginx:latest
+    ports:
+      - 80:80
+      - 443:443
+    environment:
+      - ENV=production
+      - DEBUG=false
+
+  database:
+    image: postgres:14
+    environment:
+      - POSTGRES_DB=myapp
+      - POSTGRES_USER=admin
\ No newline at end of file
diff --git a/internal/gitlab/testdata/changed_line/last_line_changed/formatted.yaml b/internal/gitlab/testdata/changed_line/last_line_changed/formatted.yaml
new file mode 100644
index 0000000..89e9d23
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/last_line_changed/formatted.yaml
@@ -0,0 +1,3 @@
+line1: ok
+line2: ok
+line3: spaced
diff --git a/internal/gitlab/testdata/changed_line/last_line_changed/original.yaml b/internal/gitlab/testdata/changed_line/last_line_changed/original.yaml
new file mode 100644
index 0000000..15ed2cc
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/last_line_changed/original.yaml
@@ -0,0 +1,3 @@
+line1: ok
+line2: ok
+line3:   spaced
diff --git a/internal/gitlab/testdata/changed_line/no_lines_changed/formatted.yaml b/internal/gitlab/testdata/changed_line/no_lines_changed/formatted.yaml
new file mode 100644
index 0000000..5851568
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/no_lines_changed/formatted.yaml
@@ -0,0 +1,2 @@
+a: b
+c: d
diff --git a/internal/gitlab/testdata/changed_line/no_lines_changed/original.yaml b/internal/gitlab/testdata/changed_line/no_lines_changed/original.yaml
new file mode 100644
index 0000000..5851568
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/no_lines_changed/original.yaml
@@ -0,0 +1,2 @@
+a: b
+c: d
diff --git a/internal/gitlab/testdata/changed_line/original.yaml b/internal/gitlab/testdata/changed_line/original.yaml
new file mode 100644
index 0000000..8c237a5
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/original.yaml
@@ -0,0 +1,18 @@
+# Example configuration file
+version: 1.0
+
+services:
+  web:
+    image:   nginx:latest
+    ports:
+      -  80:80
+      - 443:443
+    environment:
+      - ENV=production
+      - DEBUG=false
+
+  database:
+    image: postgres:14
+    environment:
+      - POSTGRES_DB=myapp
+      - POSTGRES_USER=admin
\ No newline at end of file
diff --git a/internal/gitlab/testdata/changed_line/single_line_changed/formatted.yaml b/internal/gitlab/testdata/changed_line/single_line_changed/formatted.yaml
new file mode 100644
index 0000000..6420006
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/single_line_changed/formatted.yaml
@@ -0,0 +1,3 @@
+line1: ok
+line2: spaced
+line3: ok
diff --git a/internal/gitlab/testdata/changed_line/single_line_changed/original.yaml b/internal/gitlab/testdata/changed_line/single_line_changed/original.yaml
new file mode 100644
index 0000000..ed56226
--- /dev/null
+++ b/internal/gitlab/testdata/changed_line/single_line_changed/original.yaml
@@ -0,0 +1,3 @@
+line1: ok
+line2:   spaced
+line3: ok