diff --git a/README.md b/README.md
index 19a1e03..c8f7942 100644
--- a/README.md
+++ b/README.md
@@ -53,3 +53,5 @@ parse(diffStr)
     ]
   }
 ```
+
+If the diff includes a similarity index line (from a detected copy or rename), the `similarity` property will be set, and will be a number.
diff --git a/src/diff.pegjs b/src/diff.pegjs
index b54e73f..e6194ab 100644
--- a/src/diff.pegjs
+++ b/src/diff.pegjs
@@ -43,6 +43,21 @@
     patch.binary = !!binary
     return patch
   }
+
+  function postProcessSimilarityDiff (rename_or_copy, similarity_index, old_file, new_file, file_modes, patch) {
+    const diff = {
+      oldPath: old_file,
+      newPath: new_file,
+      hunks: patch ? patch.hunks : [],
+      status: rename_or_copy === 'copy' ? 'copied' : 'renamed',
+      similarity: similarity_index,
+    }
+    if (file_modes) {
+      diff.oldMode = file_modes.old_mode
+      diff.newMode = file_modes.new_mode
+    }
+    return diff
+  }
 }
 
 diffs
@@ -50,6 +65,7 @@ diffs
 
 diff
   = binary_merge_conflict_diff
+  / rename_or_copy_diff
   / merge_conflict_diff
   / unmerged_path
   / binary_diff
@@ -67,6 +83,13 @@ regular_diff
 binary_merge_conflict_diff
   = path:merge_conflict_header_line index_line binary_declaration { return postProcessMergeConflictDiff(path, undefined, true) }
 
+rename_or_copy_diff
+  = rename_or_copy_diff_header_line modes:changed_file_modes? similarity:similarity_index copy_from:rename_copy_from copy_to:rename_copy_to
+    index_modes:index_line? patch:patch?
+  {
+    return postProcessSimilarityDiff(copy_from.operation, similarity, copy_from.file, copy_to.file, modes || index_modes, patch)
+  }
+
 merge_conflict_diff
   = path:merge_conflict_header_line index_line patch:patch? { return postProcessMergeConflictDiff(path, patch) }
 
@@ -127,9 +150,15 @@ hunk_line
 diff_header_line
   = 'diff ' options:TEXT_NO_SPACES ' ' file_name:file_name_str NL { return {file_name} }
 
+rename_or_copy_diff_header_line
+  = 'diff ' options:TEXT_NO_SPACES ' ' files_unused:TEXT NL
+
 file_name_str
   = str:TEXT { return str.substr(str.length/2 + 1) }
 
+similarity_index
+  = 'similarity index ' idx:NUMBER '%' NL { return idx }
+
 file_mode_section
   = explicit_file_modes:(new_or_deleted_file_mode / changed_file_modes)? index_file_modes:index_line? { return explicit_file_modes || index_file_modes }
 
@@ -142,6 +171,12 @@ new_or_deleted_file_mode
 changed_file_modes
   = 'old mode ' old_mode:TEXT NL 'new mode ' new_mode:TEXT NL { return {old_mode, new_mode} }
 
+rename_copy_from
+  = operation:('rename' / 'copy') ' from ' file:TEXT NL { return {operation, file} }
+
+rename_copy_to
+  = operation:('rename' / 'copy') ' to ' file:TEXT NL { return {operation, file} }
+
 index_line
   = 'index ' TEXT_NO_SPACES ' ' file_mode:TEXT NL { return {old_mode: file_mode, new_mode: file_mode} }
   / 'index ' TEXT_NO_SPACES NL
diff --git a/test/diff.test.js b/test/diff.test.js
index 03f3280..a8a847f 100644
--- a/test/diff.test.js
+++ b/test/diff.test.js
@@ -543,6 +543,79 @@ exports.testNoPatch = function(test) {
   test.done()
 }
 
+exports.testRenameCopy = function(test) {
+  var str = dedent`
+  diff --git old/file.png new/file.png
+  similarity index 90%
+  rename from old/file.png
+  rename to new/file.png
+  diff --git copy/file.png copy/file2.png
+  similarity index 100%
+  copy from copy/file.png
+  copy to copy/file2.png
+  `
+
+  const output = diff.parse(str)
+  assert.deepEqual(output, [
+    {
+      oldPath: 'old/file.png',
+      newPath: 'new/file.png',
+      status: 'renamed',
+      similarity: 90,
+      hunks: [],
+    },
+    {
+      oldPath: 'copy/file.png',
+      newPath: 'copy/file2.png',
+      status: 'copied',
+      similarity: 100,
+      hunks: [],
+    },
+  ])
+  test.done()
+}
+
+exports.testRenameWithChangedLinesAndModeChange = function(test) {
+  var str = dedent`
+  diff --git file.txt rename-file.txt
+  old mode 100644
+  new mode 100755
+  similarity index 76%
+  rename from file.txt
+  rename to rename-file.txt
+  index 471a7b8..3e32ec2
+  --- file.txt
+  +++ rename-file.txt
+  @@ -1,4 +1,5 @@
+   foo
+   bar
+   baz
+  +qux
+
+  `;
+
+  const output = diff.parse(str)
+  assert.deepEqual(output, [
+    {
+      oldPath: "file.txt",
+      newPath: "rename-file.txt",
+      oldMode: "100644",
+      newMode: "100755",
+      status: "renamed",
+      similarity: 76,
+      hunks: [{
+        oldStartLine: 1,
+        oldLineCount: 4,
+        newStartLine: 1,
+        newLineCount: 5,
+        heading: '',
+        lines: [' foo', ' bar', ' baz', '+qux']
+      }]
+    }
+  ]);
+  test.done()
+}
+
 exports.testMergeConflictNoPatch = function(test) {
   var str = dedent`
   diff --cc file-0.txt