In a prospective study of 24 consecutive patients with posttraumatic instability of the metacarpophalangeal joint of the thumb, clinical examination including instability tests and palpation of displaced ulnar collateral ligaments was used to separate the patients into two groups--nondisplaced and displaced ruptures. Palpable displaced ruptures were treated surgically, whereas nonpalpable ruptures were interpreted as nondisplaced and were treated with plaster, irrespective of the instability. At follow-up 1 year later both groups showed similar results with respect to stability, strength, and function. Our results indicate that a clinical examination with palpation of the torn ligament ends identifies displaced ruptures of the ulnar collateral ligament, i.e., those cases needing surgery. Nondisplaced ruptures might be treated nonoperatively.