diff --git a/includes/lru.inc b/includes/lru.inc
new file mode 100644
index 0000000..be26869
--- /dev/null
+++ b/includes/lru.inc
@@ -0,0 +1,61 @@
+<?php
+
+/**
+ * Provides an in-memory least recently used cache.
+ */
+
+/**
+ * PHP implementation of last recently used caching.
+ *
+ * Use this class when multiple objects of the same type will be loaded into
+ * memory during a single request, in place of usual static caching. This
+ * ensures memory usage can only reach a certain point. This class does not
+ * support persistent caching in the database across requests. 
+ */
+class LRU extends ArrayObject {
+
+  /**
+   * The number of slots in the cache.
+   */
+  protected $slots;
+
+  /**
+   * Retrieval stats.
+   */
+  protected $positions = array();
+
+  /**
+   * Constructor.
+   *
+   * @param $slots
+   *   The number of slots to make available.
+   */
+  function __construct($slots = 50) {
+    $this->slots = $slots;
+  }
+
+  function offsetGet($offset) {
+    if (parent::offsetExists($offset)) {
+      // If the item is not at the end of the positions array, move it
+      // there.
+      if (end($this->positions) != $offset) {
+        $current_position = array_search($offset, $this->positions);
+        unset($this->positions[$current_position]);
+        $this->positions[] = $offset;
+      }
+    }
+    return parent::offsetGet($offset);
+  }
+  function offsetSet($offset, $value) {
+    parent::offsetSet($offset, $value);
+    if ($this->slots > 0) {
+      $this->slots--;
+    }
+    else {
+      // Replace the item in first position in the array.
+      $key = array_shift($this->positions);
+      $this->offsetUnset($key);
+    }
+    $this->positions[] = $offset;
+  }
+}
diff --git a/modules/simpletest/simpletest.info b/modules/simpletest/simpletest.info
index 54b020d..ffbfcc0 100644
--- a/modules/simpletest/simpletest.info
+++ b/modules/simpletest/simpletest.info
@@ -23,6 +23,7 @@ files[] = tests/filetransfer.test
 files[] = tests/form.test
 files[] = tests/graph.test
 files[] = tests/image.test
+files[] = tests/lru.test
 files[] = tests/lock.test
 files[] = tests/mail.test
 files[] = tests/menu.test
diff --git a/modules/simpletest/tests/lru.test b/modules/simpletest/tests/lru.test
new file mode 100644
index 0000000..950dcba
--- /dev/null
+++ b/modules/simpletest/tests/lru.test
@@ -0,0 +1,64 @@
+<?php
+
+/**
+ * @file
+ * Provides unit tests for lru.inc.
+ */
+
+/**
+ * Unit tests for the in-memory LRU cache.
+ */
+class LRUUnitTest extends DrupalUnitTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'LRU',
+      'description' => 'LRU in-memory cache LRU tests.',
+      'group' => 'System',
+    );
+  }
+
+  /**
+   * Test the LRU cache.
+   */
+  function testLRUCache() {
+    // Manually require the file to avoid a registry lookup.
+    require_once DRUPAL_ROOT . '/includes/lru.inc';
+    // Create an in-memory LRU cache with 5 slots.
+    $this->lru = new LRU(5);
+    $this->lru['banana'] = 'banana';
+    // Confirm the item is in the cache.
+    $this->assertOffset('banana');
+    // Now add five more items to the cache.
+    foreach (array('apple', 'mango', 'grapefruit', 'kiwi', 'orange') as $fruit) {
+      $this->lru[$fruit] = $fruit;
+      $this->assertOffset($fruit);
+    }
+    // When 'orange' was added, it should have remove 'banana' from the cache.
+    $this->assertNoOffset('banana');
+    // Apple should be in the first position of the array, request it from the
+    // cache. This should move it to the front of the array, and put the mango
+    // into the LRU position.
+    $apple = $this->lru['apple'];
+    $this->assertEqual($apple, 'apple');
+    // Add banana back to the cache.
+    $this->lru['banana'] = 'banana';
+    // Confirm that mango was removed.
+    $this->assertNoOffset('mango');
+    // Confirm that the other fruits are cached.
+    foreach (array('apple', 'grapefruit', 'kiwi', 'orange') as $fruit) {
+      $this->assertOffset($fruit);
+    }
+  }
+
+  /**
+   * Assert that an item is in the LRU cache.
+   */
+  function assertOffset($offset) {
+    $this->assertTrue(isset($this->lru[$offset]), t('@item exists in the LRU cache.', array('@item' => $offset)));
+  }
+
+  function assertNoOffset($offset) {
+    $this->assertFalse(isset($this->lru[$offset]), t('@item does not exist in the LRU cache.', array('@item' => $offset)));
+  }
+}
+