Fifteen adult patients with clavicular nonunions were evaluated over a 10-year period. Fourteen of these patients were treated initially with a "figure-of-eight" clavicle strap. Seven presented with narrowing of the costoclavicular space, and symptoms of intermittent brachial plexus impingement or thoracic outlet syndrome. Two of these resulted from resection of the mid-portion of the clavicle with subsequent impingement from hypertrophy of the lateral stump. Most commonly, hypertrophic nonunion of the clavicle caused the thoracic outlet syndrome, frequently many months after the initial injury. This delayed onset led to diagnostic confusion with other causes of radiating upper extremity pain, including cervical disc disease. Treatment of the adult clavicle fracture is different from that of the child with potential for greater functional impairment. The "figure-of-eight" harness frequently does not provide pain relief or adequate reduction of the fracture. If an acutely displaced fracture in an adult cannot be reduced and held by closed casting technique, open reduction and internal fixation may occasionally be indicated. We have found transcortical fixation by locking Knowles pins to be effective for treating hypertrophic nonunions, as well as for unreducible acute fractures. Atrophic nonunions require plate fixation and autologous grafting. Nonunion of the clavicle has been among the most responsive of nonunions in our experience, since all fractures treated by adequate reduction and fixation healed promptly.(ABSTRACT TRUNCATED AT 250 WORDS)