There are few long-term follow-up reports concerning the treatment of thumb duplication. We reviewed the treatment of 19 of 74 patients treated at our institution between 1956 and 2002. The average follow-up was 22 (range, 7 years to 35 years) years. Satisfactory function was achieved in 18 thumbs and cosmesis in 12 thumbs. Six thumbs were cold-intolerant at this late follow-up. Pinch strength was similar to the contralateral normal thumbs. Collateral ligament repair did not significantly contribute to joint stability. An objective method of postoperative evaluation showed good results in five, fair results in 12 and poor results in two thumbs.