The abductor digiti minimi (ADM) opponensplasty is used widely as a standard technique of opponensplasty for congenital hypoplastic thumb. Functional results, however, are not always satisfactory in cases of marked laxity of the thumb metacarpophalangeal (MP) joint. The authors have developed a modified procedure of ADM opponensplasty to stabilize the thumb MP joint and to obtain appropriate opponens function. The first step of their modification is to retain the maximum length of transferred ADM muscle. The origin of the ADM is shifted radially and is reattached to the transverse carpal ligament. In our modification the ADM tendon is passed underneath the extensor pollicis longus tendon and is sutured to the adductor pollicis tendon at the ulnar side of the thumb MP joint. By this modified anchoring point the transferred ADM tendon runs across the ulnar side of the thumb MP joint and is expected to act as if it were an ulnar collateral ligament.