A common finding in tennis elbow is pain in the region of the lateral epicondyle during resisted extension of the middle finger (Maudsley's test). We hypothesized that the pain is due to disease in the extensor digitorum communis muscle, rather than to compression of the radial nerve or disease within extensor carpi radialis brevis. Thirteen human forearm specimens were examined. It was found that the extensor digitorum communis was separable into four parts. The part to the middle finger originated from the lateral epicondyle, but the muscle slips to the other fingers originated more distally. Pain ratings were measured in ten patients diagnosed with lateral epicondylitis during isometric finger and wrist extension tests. The results confirmed the high prevalence of a positive Maudsley's test in lateral epicondylitis, and also that the patients with tenderness at the site of origin of the extensor digitorum communis slip to the middle finger had the greatest pain during middle finger extension. These anatomical and clinical findings clarify the anatomy of extensor digitorum communis, and suggest that this muscle forms the basis for the Maudsley's test. The muscle may play a greater role in tennis elbow than previously appreciated.