Closed ruptures of the extensor pollicis longus tendon usually appear as a consequence of fractures of the wrist joint or the carpal bones or ensue from polyarthritic changes or result from a process of degeneration. Mechanical injury of the tendon is quite rare but can be observed after direct trauma or after operative treatment of a distal fracture of the radius. Closed traumatic ruptures of the extensor pollicis longus tendon in the absence of pathological changes are--in spite of the frequency of rotation injuries of the forearm--very rare. The patient in the case described is a 45-year-old locksmith whose forearm and hand had been caught in a lathe. This led to sudden, extreme rotation and pronation of the hand. Clinically distinct signs of a rupture of the extensor pollicis longus tendon could be seen. During operation a tear of the muscular portion of the long extensor tendon of the thumb was found. The function of extension was restored by transfer of the extensor indicis tendon.