diff --git a/History.md b/History.md
index f514c1b9..0b568ef9 100644
--- a/History.md
+++ b/History.md
@@ -1,6 +1,7 @@
 unreleased
 ==========
 
+  * Add `genid` option to generate custom session IDs
   * Add `saveUninitialized` option to control saving uninitialized sessions
   * Add `unset` option to control unsetting `req.session`
   * deps: buffer-crc32@0.2.3
diff --git a/README.md b/README.md
index 3bb68e86..fa3a64af 100644
--- a/README.md
+++ b/README.md
@@ -34,12 +34,27 @@ middleware _before_ `session()`.
   - `secret` - session cookie is signed with this secret to prevent tampering.
   - `cookie` - session cookie settings.
     - (default: `{ path: '/', httpOnly: true, secure: false, maxAge: null }`)
+  - `genid` - function to call to generate a new session ID. (default: uses `uid2` library)
   - `rolling` - forces a cookie set on every response. This resets the expiration date. (default: `false`)
   - `resave` - forces session to be saved even when unmodified. (default: `true`)
   - `proxy` - trust the reverse proxy when setting secure cookies (via "x-forwarded-proto" header). When set to `true`, the "x-forwarded-proto" header will be used. When set to `false`, all headers are ignored. When left unset, will use the "trust proxy" setting from express. (default: `undefined`)
   - `saveUninitialized` - forces a session that is "uninitialized" to be saved to the store. A session is uninitialized when it is new but not modified. This is useful for implementing login sessions, reducing server storage usage, or complying with laws that require permission before setting a cookie. (default: `true`)
   - `unset` - controls result of unsetting `req.session` (through `delete`, setting to `null`, etc.). This can be "keep" to keep the session in the store but ignore modifications or "destroy" to destroy the stored session. (default: `'keep'`)
 
+#### options.genid
+
+Generate a custom session ID for new sessions. Provide a function that returns a string that will be used as a session ID. The function is given `req` as the first argument if you want to use some value attached to `req` when generating the ID.
+
+**NOTE** be careful you generate unique IDs so your sessions do not conflict.
+
+```js
+app.use(session({
+  genid: function(req) {
+    return genuuid(); // use UUIDs for session IDs
+  },
+  secret: 'keyboard cat'
+}))
+```
 
 #### Cookie options
 
diff --git a/index.js b/index.js
index db20d65c..cfc7ed3d 100644
--- a/index.js
+++ b/index.js
@@ -72,6 +72,12 @@ function session(options){
     , storeReady = true
     , rollingSessions = options.rolling || false;
 
+  var generateId = options.genid || generateSessionId;
+
+  if (typeof generateId !== 'function') {
+    throw new TypeError('genid option must be a function');
+  }
+
   // TODO: switch default to false on next major
   var resaveSession = options.resave === undefined
     ? true
@@ -96,7 +102,7 @@ function session(options){
 
   // generates the new session
   store.generate = function(req){
-    req.sessionID = uid(24);
+    req.sessionID = generateId(req);
     req.session = new Session(req);
     req.session.cookie = new Cookie(cookie);
   };
@@ -276,6 +282,17 @@ function session(options){
   };
 };
 
+/**
+ * Generate a session ID for a new session.
+ *
+ * @return {String}
+ * @api private
+ */
+
+function generateSessionId(sess) {
+  return uid(24);
+}
+
 /**
  * Hash the given `sess` object omitting changes to `.cookie`.
  *
diff --git a/test/session.js b/test/session.js
index 78e52517..0c8ed37f 100644
--- a/test/session.js
+++ b/test/session.js
@@ -186,6 +186,55 @@ describe('session()', function(){
     })
   })
 
+  describe('genid option', function(){
+    it('should reject non-function values', function(){
+      session.bind(null, { genid: 'bogus!' }).should.throw(/genid.*must/);
+    });
+
+    it('should provide default generator', function(done){
+      request(app)
+      .get('/')
+      .expect('set-cookie', /connect\.sid=s%3A([^\.]{24})\./i)
+      .expect(200, done);
+    });
+
+    it('should allow custom function', function(done){
+      var app = express()
+        .use(cookieParser())
+        .use(session({ genid: function(){ return 'a' }, secret: 'keyboard cat', cookie: { maxAge: min }}))
+        .use(respond);
+
+      request(app)
+      .get('/')
+      .expect('set-cookie', /connect\.sid=s%3Aa\./i)
+      .expect(200, done);
+    });
+
+    it('should encode unsafe chars', function(done){
+      var app = express()
+        .use(cookieParser())
+        .use(session({ genid: function(){ return '%' }, secret: 'keyboard cat', cookie: { maxAge: min }}))
+        .use(respond);
+
+      request(app)
+      .get('/')
+      .expect('set-cookie', /connect\.sid=s%3A%25\./i)
+      .expect(200, done);
+    });
+
+    it('should provide req argument', function(done){
+      var app = express()
+        .use(cookieParser())
+        .use(session({ genid: function(req){ return req.url }, secret: 'keyboard cat', cookie: { maxAge: min }}))
+        .use(respond);
+
+      request(app)
+      .get('/foo')
+      .expect('set-cookie', /connect\.sid=s%3A%2Ffoo\./i)
+      .expect(200, done);
+    });
+  });
+
   describe('key option', function(){
     it('should default to "connect.sid"', function(done){
       request(app)