Over a twenty-two-year period, 237 patients (261 hands) with duplication of the thumb were seen in the Hand Clinic of Osaka University Hospital. Two groups were identified: Group A, 141 patients without previous surgical treatment, and Group B, ninety-six patients with residual deformity despite previous surgical treatment. Using a modification of Wassel's classification, seven types of deformity were defined. In Group A these types were identified on the basis of the observed duplications of bone and soft tissue. In all but ten of the Group-B patients preoperative roentgenograms were not available and the type of deformity had to be deduced from the residual duplicated bone, the surgical scar, and the residual deformity. Surgery, performed on 193 hands (125 in Group A and sixty-eight in Group B), attempted to restore normal anatomical relationships. The results could be evaluated in 130 hands according to the range of motion, joint stability, and alignment of the remaining thumb after an average follow-up of 35.0 months. According to the rating system described, the results were rated as good in 75.5 per cent, fair in 20.2 per cent, and poor in 4.3 per cent of the ninety-four hands in Group-A patients who were followed. In the thirty-six hands of Group-B patients who could be followed, the preoperative and postoperative scores were compared. Thirteen were not improved while the other twenty-three, sixteen improved from fair to good and seven improved from poor to fair, to give a good result in 63.9 per cent of the Group-B patients who were followed. The results in these 130 Group-A and B hands emphasize the importance of providing muscle balance and, in young patients, of performing an arthroplasty of the interphalangeal or metacarpophalangeal joint when indicated, although arthrodesis was indicated as a salvage operation for Group-B patients who were more than fifteen years old.