CONTEXT: Lateral epicondylalgia, pain at the lateral elbow, is commonly associated with extensor carpi radialis brevis tendinopathy. The radial head, which abuts the extensor tendons and is elliptical in shape, may affect the extensor tendons during pronation of the forearm. Cadaverous studies have shown that the radial head may act as a cam in pronation, to offer a mechanical advantage to the common extensor tendon and to mitigate load on the origin of the extensor carpi radialis brevis tendon. OBJECTIVE: To determine the effect of radial-head position on the wrist-extensor tendons in vivo. DESIGN: Repeated-measures design. SETTING: Laboratory. PARTICIPANTS: 22 participants (12 male, 2 left-handed). MAIN OUTCOME MEASURES: Distance (mm) from subcutaneous fascia to radial head measured by ultrasound. RESULTS: The radial head in supination was significantly deeper than either pronation or midprone, indicating a smaller cam effect in supination. CONCLUSION: The authors recommend that the effect of radial-head position and its relationship to the area of tendon pathology be considered clinically in the rehabilitation of patients suffering from lateral epicondylalgia.