This study reports on outcomes of the flexor digitorum superficialis tendon transfer from the ring finger in Type II and IIIA hypoplastic thumbs. We included 22 patients with 27 involved hands: 16 Type II thumbs and five Type IIIA treated by transfer and six non-operated Type II thumbs. The outcomes were assessed by range of motion, strength, sensibility, joint stability and patient/parent satisfaction. Compared with normative data, the range of motion was diminished in all patient groups. Opposition strength in operated Type II thumbs was significantly better than in non-operated thumbs. Grip strength, pinch strength, tripod strength and key pinch strength were approximately 50% of normal in Type II thumbs and 35% in Type IIIA thumbs. Metacarpophalangeal joint stability was restored in all operated Type II thumbs and in 40% of Type IIIA thumbs. We conclude that the flexor digitorum superficialis tendon transfer of the ring finger is a good functioning opponensplasty in both Type II and IIIA thumbs. The transfer provides excellent stability of the metacarpophalangeal joint in Type II thumbs.