diff --git a/source-map-support.js b/source-map-support.js
index a9360bd..bfddefc 100644
--- a/source-map-support.js
+++ b/source-map-support.js
@@ -2,38 +2,58 @@ var SourceMapConsumer = require('source-map').SourceMapConsumer;
 var path = require('path');
 var fs = require('fs');
 
-exports.mapSourcePosition = mapSourcePosition = function(cache, position) {
+// Can be overridden by the retrieveSourceMap option to install. Takes a
+// generated source filename; returns a {map, optional url} object, or null if
+// there is no source map.  The map field may be either a string or the parsed
+// JSON object (ie, it must be a valid argument to the SourceMapConsumer
+// constructor).
+var retrieveSourceMap = function (source) {
+  if (!fs.existsSync(source))
+    return null;
+
+  // Get the URL of the source map
+  var fileData = fs.readFileSync(source, 'utf8');
+  var match = /\/\/[#@]\s*sourceMappingURL=(.*)\s*$/m.exec(fileData);
+  if (!match) return null;
+  var sourceMappingURL = match[1];
+
+  // Read the contents of the source map
+  var sourceMapData;
+  var dataUrlPrefix = "data:application/json;base64,";
+  if (sourceMappingURL.slice(0, dataUrlPrefix.length).toLowerCase() == dataUrlPrefix) {
+    // Support source map URL as a data url
+    sourceMapData = new Buffer(sourceMappingURL.slice(dataUrlPrefix.length), "base64").toString();
+  }
+  else {
+    // Support source map URLs relative to the source URL
+    var dir = path.dirname(source);
+    sourceMappingURL = path.resolve(dir, sourceMappingURL);
+
+    if (fs.existsSync(sourceMappingURL)) {
+      sourceMapData = fs.readFileSync(sourceMappingURL, 'utf8');
+    }
+  }
+
+  if (!sourceMapData) {
+    return null;
+  }
+
+  return {
+    url: sourceMappingURL,
+    map: sourceMapData
+  };
+};
+
+var mapSourcePosition = exports.mapSourcePosition = function(cache, position) {
   var sourceMap = cache[position.source];
-  if (!sourceMap && fs.existsSync(position.source)) {
-    // Get the URL of the source map
-    var fileData = fs.readFileSync(position.source, 'utf8');
-    var match = /\/\/[#@]\s*sourceMappingURL=(.*)\s*$/m.exec(fileData);
-    if (!match) return position;
-    var sourceMappingURL = match[1];
-
-    // Read the contents of the source map
-    var sourceMapData;
-    var dataUrlPrefix = "data:application/json;base64,";
-    if (sourceMappingURL.slice(0, dataUrlPrefix.length).toLowerCase() == dataUrlPrefix) {
-      // Support source map URL as a data url
-      sourceMapData = new Buffer(sourceMappingURL.slice(dataUrlPrefix.length), "base64").toString();
-    }
-    else {
-      // Support source map URLs relative to the source URL
-      var dir = path.dirname(position.source);
-      sourceMappingURL = path.resolve(dir, sourceMappingURL);
-
-      if (fs.existsSync(sourceMappingURL)) {
-        sourceMapData = fs.readFileSync(sourceMappingURL, 'utf8');
-      }
-    }
-
-    if (sourceMapData) {
-      sourceMap = {
-        url: sourceMappingURL,
-        map: new SourceMapConsumer(sourceMapData)
+  if (!sourceMap) {
+    // Call the (overrideable) retrieveSourceMap function to get the source map.
+    var urlAndMap = retrieveSourceMap(position.source);
+    if (urlAndMap) {
+      sourceMap = cache[position.source] = {
+        url: urlAndMap.url,
+        map: new SourceMapConsumer(urlAndMap.map)
       };
-      cache[position.source] = sourceMap;
     }
   }
 
@@ -47,13 +67,15 @@ exports.mapSourcePosition = mapSourcePosition = function(cache, position) {
     // better to give a precise location in the compiled file than a vague
     // location in the original file.
     if (originalPosition.source !== null) {
-      originalPosition.source = path.resolve(path.dirname(sourceMap.url), originalPosition.source);
+      if (sourceMap.url) {
+        originalPosition.source = path.resolve(path.dirname(sourceMap.url), originalPosition.source);
+      }
       return originalPosition;
     }
   }
 
   return position;
-}
+};
 
 // Parses code generated by FormatEvalOrigin(), a function inside V8:
 // https://code.google.com/p/v8/source/browse/trunk/src/messages.js
@@ -162,6 +184,11 @@ exports.install = function(options) {
   var installHandler = 'handleUncaughtExceptions' in options ?
     options.handleUncaughtExceptions : true;
 
+  // Allow source maps to be found by methods other than reading the files
+  // directly from disk.
+  if (options.retrieveSourceMap)
+    retrieveSourceMap = options.retrieveSourceMap;
+
   // Provide the option to not install the uncaught exception handler. This is
   // to support other uncaught exception handlers (in test frameworks, for
   // example). If this handler is not installed and there are no other uncaught