Of eithty-six patients with collateral ligament instability of the metacarpophalangeal joint of the thumb, sixty-six (77 per cent) had ulnar and twenty (23 per cent) had radial instability, while in addition twenty-four had fractures and twenty-nine had volar subluxation of the proximal phalanx. Of the sixty-nine patients operated on, sixty-five had an abnormality of the ulnar or radial collateral ligament proper or of its attachment; two, subluxation of the extensor pollicis longus; and two, stretching of the adductor expansion. Stability was restored by collateral ligament repair or reconstruction, by fixation of a fracture fragment with a Kirschner wire, or by arthrodesis in all but five thumbs. Of the five patients who did not have stability of the metacarpophalangeal joint following surgery, three had had the collateral ligament sutured more than three weeks after injury and two had had reconstruction of the collateral ligament using a tendon graft. Early surgical treatment is recommended for all patients with post-traumatic instability of the metacarpophalangeal joint greater than 45 degrees and for those with volar subluxation of the proximal phalanx or a displaced fracture of the base of the proximal phalanx. Collateral ligament repair is indicated for patients operated on within three weeks of injury, and reconstruction of the ligament by means of a tendon graft is recommended for those treated more than three weeks after injury.