We report a retrospective study of avulsion fractures of the ulnar collateral ligament of the thumb metacarpophalangeal joint that were treated nonsurgically. The study included 30 patients who answered a questionnaire. None of the patients underwent surgery after treatment. The average follow-up interval was 3.1 years (range, 1-5.2 years). All 30 patients were satisfied with the treatment and outcome. No patient had a permanent job change or inability to participate in recreational activities. Nineteen patients had no pain on motion of the thumb. Of the 20 patients (67%) who agreed to a follow-up examination, key pinch and grip strength were not statistically different between the injured and noninjured sides. Three patients had instability on stress testing. There was a 25% nonunion rate. While not statistically significant, symptomatic patients tended to have slightly larger fragments (5.4 mm x 1.7 mm vs 3.2 mm x 1.4 mm on anterioposterior x-rays) and greater initial rotation (26 degrees vs 24 degrees) of the fracture than asymptomatic patients. Despite the nonunion rate and the patients with clinical instability, all 30 patients were satisfied with their treatment.