Several apparently conflicting mechanisms have been proposed to explain the seemingly spontaneous delayed rupture of the extensor pollicis longus tendon (EPL). The following case, the first of its kind of which we are aware, may help to clarify the relationships between these mechanisms. Traumatic hyperextension of a patient's wrist brought the styloid process of the third metacarpal into contact with Lister's tubercle, fracturing it. Chronic chafing of the EPL on the nonunited fracture's jagged surface seemed to cause its spontaneous rupture 18 months later. The long period between accident and rupture is evidence that rupture was not caused by crush injury. Because a fracture of Lister's tubercle will not normally be visible on radiographs, after accidents in which this may have occurred or when the EPL ruptures more than 3 months after injury, we recommend that special radiographs of Lister's tubercle be taken to determine if such a fracture exists.