diff --git a/lib/parser.js b/lib/parser.js
index a904e5d..be4fe45 100755
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -153,13 +153,20 @@ readFileContent = function (code, filename, parser, localFilename) {
 		type = ext;
 		sourceUrl = localFilename;
 		if (parser.transform) {
-			code = deferred(parser.transform(filename, code))(function (code) {
-				if (code == null) {
+			code = deferred(parser.transform(filename, code))(function (data) {
+				var code;
+				if (data == null) {
 					throw customError("Provided transform callback must return code string",
 						'INVALID_TRANSFORM');
 				}
-				if (ext !== '.js') return String(code);
-				return stripBOM(String(code)).replace(sheBangRe, '//$1\n');
+				code = data.code || data;
+				if (ext !== '.js') code = String(code);
+				else code = stripBOM(String(code)).replace(sheBangRe, '//$1\n');
+				if (parser.sourceMap && data.sourceMap) {
+					code += '//# sourceMappingURL=data:application/json;base64,' +
+						new Buffer(data.sourceMap).toString('base64') + '\n';
+				}
+				return code;
 			});
 		} else if (ext === '.js') {
 			code = stripBOM(code).replace(sheBangRe, '//$1\n');