diff --git a/lib/directoryindex.js b/lib/directoryindex.js
new file mode 100644
index 0000000..131432a
--- /dev/null
+++ b/lib/directoryindex.js
@@ -0,0 +1,68 @@
+var _ = require('underscore');
+var fs = require('fs');
+var path = require('path');
+var util = require('util');
+var EventEmitter = require('events').EventEmitter;
+
+module.exports = DirectoryIndex;
+util.inherits(DirectoryIndex, EventEmitter)
+function DirectoryIndex(dir) {
+    this.filepath = dir;
+
+    // Initialize and continually update the directory index.
+    this.update = _(this.update).bind(this);
+    process.nextTick(this.update)
+    fs.watchFile(dir, this.update);
+}
+
+DirectoryIndex.prototype.getList = function(callback) {
+    if (!this.list) this.once('updated', done);
+    else done.call(this);
+
+    function done() {
+        if (this.err) callback(this.err);
+        else callback(null, this.list);
+    }
+};
+
+DirectoryIndex.prototype.getID = function(id, callback) {
+    if (!this.list) this.once('updated', done);
+    else done.call(this);
+
+    function done() {
+        if (this.err) callback(this.err);
+        else if (!this.list[id]) callback(new Error('Tileset not found'));
+        else callback(null, this.list[id]);
+    }
+}
+
+DirectoryIndex.prototype.update = function() {
+    var index = this;
+    fs.readdir(index.filepath, function(err, files) {
+        index.err = err;
+        if (!err) {
+            for (var result = index.list = {}, i = 0; i < files.length; i++) {
+                var name = files[i].match(/^([\w-]+)\.mbtiles$/);
+                if (name) {
+                    result[name[1]] = 'mbtiles://' + path.join(index.filepath, name[0]);
+                }
+            }
+            index.list = result;
+        }
+        index.emit('updated');
+    });
+};
+
+var cache = DirectoryIndex.cache = {};
+DirectoryIndex.create = function(filepath) {
+    if (!cache[filepath]) {
+        var oldpath = filepath;
+        filepath = path.resolve(filepath);
+        if (!cache[filepath]) {
+            cache[filepath] = new DirectoryIndex(filepath);
+        }
+        // Avoid path.resolve() calls.
+        cache[oldpath] = cache[filepath];
+    }
+    return cache[filepath];
+};
diff --git a/lib/mbtiles.js b/lib/mbtiles.js
index 8dfdcc9..2db214b 100644
--- a/lib/mbtiles.js
+++ b/lib/mbtiles.js
@@ -10,6 +10,9 @@ var _ = require('underscore'),
     sm = new (require('sphericalmercator')),
     sqlite3 = require('sqlite3');
 
+var DirectoryIndex = require('./directoryindex');
+
+
 if (process.env.NODE_ENV === 'test') sqlite3.verbose();
 
 function noop(err) {
@@ -109,26 +112,13 @@ MBTiles.registerProtocols = function(tilelive) {
 
 // Finds all mbtiles file in the filepath and returns their tilesource URI.
 MBTiles.list = function(filepath, callback) {
-    filepath = path.resolve(filepath);
-    fs.readdir(filepath, function(err, files) {
-        if (err) return callback(err);
-        for (var result = {}, i = 0; i < files.length; i++) {
-            var name = files[i].match(/^([\w-]+)\.mbtiles$/);
-            if (name) result[name[1]] = 'mbtiles://' + path.join(filepath, name[0]);
-        }
-        return callback(null, result);
-    });
+    DirectoryIndex.create(filepath).getList(callback);
 };
 
 // Finds an mbtiles file with the given ID in the filepath and returns a
 // tilesource URI.
 MBTiles.findID = function(filepath, id, callback) {
-    filepath = path.resolve(filepath);
-    var file = path.join(filepath, id + '.mbtiles');
-    fs.stat(file, function(err, stats) {
-        if (err) return callback(err);
-        else return callback(null, 'mbtiles://' + file);
-    });
+    DirectoryIndex.create(filepath).getID(id, callback);
 };
 
 // Retrieve the schema of the current mbtiles database and inform the caller of