Eleven patients with disabling osteoarthritis at the base of the thumb were treated with a Dacron-reinforced trapezium replacement arthroplasty. Examination from 1 to 3 years later (average, 20 months) showed that all patients were satisfied, principally because of relief of pain. The range of motion and stability were satisfactory, although far from normal. Postoperative dislocation did not occur. This problem was eliminated by correcting hyperextension of the metacarpal phalangeal joint and by reinforcing the capsule over the trapezium with a buttress created by transferring half of the distal end of the flexor carpi radialis to the insertion of the extensor carpi radialis longus. None of the prostheses fractured and no bone erosion was seen on postoperative roentgenogram.