diff --git a/index.js b/index.js
index 8c02aac..19f3e5f 100644
--- a/index.js
+++ b/index.js
@@ -225,31 +225,42 @@ var parseJSON = function parseJSON(data, filename) {
  * Asynchronously walks the file list until a match is found. If
  * no matches are found, calls the callback with an error
 **/
-var readFirstFile = function readFirstFile(uri, filenames, css, cb, examinedFiles) {
+var readFirstFile = function readFirstFile(uri, filenames, options, cb, examinedFiles) {
   var filename = filenames.shift();
   examinedFiles = examinedFiles || [];
   examinedFiles.push(filename);
-  fs.readFile(filename, function(err, data) {
-    if (err) {
-      if (filenames.length) {
-        readFirstFile(uri, filenames, css, cb, examinedFiles);
+  fs.readFile(filename, (function (filename) {
+    return function(err, data) {
+      if (err) {
+        if (filenames.length) {
+          readFirstFile(uri, filenames, options, cb, examinedFiles);
+        }
+        else {
+          cb(new Error('Could not import `' + uri + '` from any of the following locations:\n  ' + examinedFiles.join('\n  ')));
+        }
       }
       else {
-        cb(new Error('Could not import `' + uri + '` from any of the following locations:\n  ' + examinedFiles.join('\n  ')));
-      }
-    }
-    else {
-      if ([ '.js', '.json', '.yml', '.yaml' ].indexOf(path.extname(filename)) !== -1) {
-        data = parseJSON(data, filename);
+        if ([ '.js', '.json', '.yml', '.yaml' ].indexOf(path.extname(filename)) !== -1) {
+          data = parseJSON(data, filename);
+        }
+
+        var contents = data.toString();
+        var tmp = options.importOnce.transformContent(filename, contents);
+        if (typeof tmp != 'undefined') {
+          contents = tmp;
+        }
+
+        cb(null, {
+          'contents': contents,
+          'file': filename
+        });
       }
-      cb(null, {
-        'contents': data.toString(),
-        'file': filename
-      });
-    }
-  });
+    };
+  })(filename));
 };
 
+var noop = function noop () {};
+
 // This is a bootstrap function for calling readFirstFile.
 var readAbstractFile = function readAbstractFile(uri, abstractName, cb) {
   var gfn = getFileNames.bind(this),
@@ -267,7 +278,7 @@ var readAbstractFile = function readAbstractFile(uri, abstractName, cb) {
     files = files.concat(gbn(uri));
   }
 
-  readFirstFile(uri, files, css, cb);
+  readFirstFile(uri, files, this.options, cb);
 };
 
 /**
@@ -299,6 +310,10 @@ var importer = function importer(uri, prev, done) {
     this.options.importOnce.css = false;
   }
 
+  if (typeof this.options.importOnce.transformContent != 'function') {
+    this.options.importOnce.transformContent = noop;
+  }
+
   // Create an import cache if it doesn't exist
   if (!this._importOnceCache) {
     this._importOnceCache = {};