The experiences with 100 operations of pollicization of the index finger in congenital deformities of the thumb in seventy-three patients are reported on. The operation is indicated in aplasia and in hypoplasia if there is no stable metacarpophalangeal joint and there are adduction contracture and poor mobility due to abnormalities of muscles and tendons. The operative technique was improved by the experiences in twelve years of use. Most important are the bone fixation and the muscle stabilization. In transformation of an index finger into a thumb it is necessary to shorten the ray, to rotate it sufficiently, and to give it the right angle of abduction. The shortening takes place in the metacarpal bone with preservation of its head: the metacarpophalangeal joint becomes now the carpometacarpal joint and the metacarpal head the new trapezium. For preventing a hyperextension deformity it is necessary to rotate the metacarpal head. For good muscle stabilization and good mobility the two interossei of the index finger have to be detached and to be fixed in the shortened position on both sides of the new thumb to the mobilized lateral slips of the dorsal aponeurosis. This new manner of muscle fixation gives a very good thenar eminence not only in respect to appearance but also in function with full or nearly full opposition and abduction. It requires a new skin incision with a dorsoradial skin flap for covering the gap between the wound edges of the former proximal phalanx broadened by the muscle masses. The excellent results show a surprising adaptation of bones and soft tissues due to function and growth—a reason for an early performance of the operation.