In a prospective randomized study that included 63 consecutive thumbs with injuries of the ulnar collateral ligament of the metacarpophalangeal (MCP) joint of the thumb, plaster cast immobilization was compared with functional treatment with a splint. The splint allowed flexion and extension of the MCP joint, but prevented ulnar and radial deviation of the thumb. The study included both operated on and nonoperated on cases where surgery was performed only when the torn ligament was regarded as displaced. Of 40 thumbs treated nonsurgically, 21 were treated with a cast and 19 with a splint. Of 23 thumbs treated surgically, 10 were immobilized postoperatively in a plaster cast and 13 were treated with the splint. At the follow-up examination after 15 (11-41) months, there was no difference between the treatment groups as regards stability, range of motion, strength of the injured thumb, and length of sick leave. However, the patients considered the splint more comfortable than plaster cast immobilization. We conclude that immobilization of the thumb after a ligamentous injury with a movable splint is strongly preferred by the patients and that the functional results of this technique are equal to plaster cast immobilization after both surgical and nonsurgical treatment.