diff --git a/lib/src/executable/options.dart b/lib/src/executable/options.dart
index 41451d61..add971d9 100644
--- a/lib/src/executable/options.dart
+++ b/lib/src/executable/options.dart
@@ -407,7 +407,7 @@ class ExecutableOptions {
   Uri sourceMapUrl(Uri url, String destination) {
     if (url.scheme.isNotEmpty && url.scheme != 'file') return url;
 
-    var path = p.canonicalize(p.fromUri(url));
+    var path = p.fromUri(url);
     return p.toUri(_options['source-map-urls'] == 'relative'
         ? p.relative(path, from: p.dirname(destination))
         : p.absolute(path));
diff --git a/lib/src/importer/filesystem.dart b/lib/src/importer/filesystem.dart
index c0e17a09..e4936679 100644
--- a/lib/src/importer/filesystem.dart
+++ b/lib/src/importer/filesystem.dart
@@ -27,7 +27,12 @@ class FilesystemImporter extends Importer {
   ImporterResult load(Uri url) {
     var path = p.fromUri(url);
     return ImporterResult(io.readFile(path),
-        sourceMapUrl: url, syntax: Syntax.forPath(path));
+        sourceMapUrl:
+            // [io.realCasePath] will short-circuit on case-sensitive
+            // filesystems anyway, but we still avoid calling it here so we
+            // don't have to re-parse the URL.
+            io.couldBeCaseInsensitive ? p.toUri(io.realCasePath(path)) : url,
+        syntax: Syntax.forPath(path));
   }
 
   DateTime modificationTime(Uri url) => io.modificationTime(p.fromUri(url));
diff --git a/lib/src/io.dart b/lib/src/io.dart
index 8524834b..aba160a5 100644
--- a/lib/src/io.dart
+++ b/lib/src/io.dart
@@ -2,6 +2,41 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'package:path/path.dart' as p;
+
+import 'io/interface.dart'
+    if (dart.library.io) 'io/vm.dart'
+    if (dart.library.js) 'io/node.dart';
+import 'utils.dart';
+
 export 'io/interface.dart'
     if (dart.library.io) 'io/vm.dart'
     if (dart.library.js) 'io/node.dart';
+
+/// Returns whether the current operating system might be case-insensitive.
+///
+/// We can't know for sure because different Mac OS systems are configured
+/// differently.
+bool get couldBeCaseInsensitive => isWindows || isMacOS;
+
+/// Returns `path` with the case updated to match the path's case on disk.
+///
+/// This only updates `path`'s basename. It always returns `path` as-is on
+/// operating systems other than Windows or Mac OS, since they almost never uses
+/// case-insensitive filesystems.
+String realCasePath(String path) {
+  // TODO(nweiz): Use an SDK function for this when dart-lang/sdk#35370 and/or
+  // nodejs/node#24942 are fixed.
+
+  if (!couldBeCaseInsensitive) return path;
+
+  var basename = p.basename(path);
+  var matches = listDir(p.dirname(path))
+      .where((realPath) => equalsIgnoreCase(p.basename(realPath), basename))
+      .toList();
+
+  // If the file doesn't exist, or if there are multiple options (meaning the
+  // filesystem isn't actually case-insensitive), return `path` as-is.
+  if (matches.length != 1) return path;
+  return matches.first;
+}
diff --git a/lib/src/io/interface.dart b/lib/src/io/interface.dart
index 498c3252..16653d65 100644
--- a/lib/src/io/interface.dart
+++ b/lib/src/io/interface.dart
@@ -33,6 +33,9 @@ Stderr get stderr => null;
 /// Whether the current process is running on Windows.
 bool get isWindows => false;
 
+/// Whether the current process is running on Mac OS.
+bool get isMacOS => false;
+
 /// Returns whether or not stdout is connected to an interactive terminal.
 bool get hasTerminal => false;
 
diff --git a/lib/src/io/node.dart b/lib/src/io/node.dart
index b7c6485b..f7cec7bb 100644
--- a/lib/src/io/node.dart
+++ b/lib/src/io/node.dart
@@ -232,6 +232,8 @@ bool get hasTerminal => _hasTerminal ?? false;
 
 bool get isWindows => _process.platform == 'win32';
 
+bool get isMacOS => _process.platform == 'darwin';
+
 bool get isNode => true;
 
 // Node seems to support ANSI escapes on all terminals.
diff --git a/lib/src/io/vm.dart b/lib/src/io/vm.dart
index e2b8c41e..849f7dc9 100644
--- a/lib/src/io/vm.dart
+++ b/lib/src/io/vm.dart
@@ -19,6 +19,8 @@ io.Stdout get stderr => io.stderr;
 
 bool get isWindows => io.Platform.isWindows;
 
+bool get isMacOS => io.Platform.isMacOS;
+
 bool get hasTerminal => io.stdout.hasTerminal;
 
 bool get isNode => false;
diff --git a/test/cli/shared/source_maps.dart b/test/cli/shared/source_maps.dart
index 94915160..fb3f032b 100644
--- a/test/cli/shared/source_maps.dart
+++ b/test/cli/shared/source_maps.dart
@@ -89,6 +89,34 @@ void sharedTests(Future<TestProcess> runSass(Iterable<String> arguments)) {
     });
   });
 
+  group("doesn't normalize file case", () {
+    setUp(() => d.file("TeSt.scss", "a {b: c}").create());
+
+    test("when loaded with the same case", () async {
+      await (await runSass(["TeSt.scss", "out.css"])).shouldExit(0);
+      expect(_readJson("out.css.map"), containsPair("sources", ["TeSt.scss"]));
+    });
+
+    test("when imported with the same case", () async {
+      await d.file("importer.scss", "@import 'TeSt.scss'").create();
+      await (await runSass(["importer.scss", "out.css"])).shouldExit(0);
+      expect(_readJson("out.css.map"), containsPair("sources", ["TeSt.scss"]));
+    });
+
+    // The following tests rely on Windows' case-insensitive filesystem.
+
+    test("when loaded with a different case", () async {
+      await (await runSass(["test.scss", "out.css"])).shouldExit(0);
+      expect(_readJson("out.css.map"), containsPair("sources", ["TeSt.scss"]));
+    }, testOn: "windows");
+
+    test("when imported with a different case", () async {
+      await d.file("importer.scss", "@import 'test.scss'").create();
+      await (await runSass(["importer.scss", "out.css"])).shouldExit(0);
+      expect(_readJson("out.css.map"), containsPair("sources", ["TeSt.scss"]));
+    }, testOn: "windows");
+  });
+
   test("includes a source map comment", () async {
     await d.file("test.scss", "a {b: c}").create();
     await (await runSass(["test.scss", "out.css"])).shouldExit(0);