One of two genetically identical twins (30 years old) had been born with two normal testes and the other with none. In the anorchic twin, preoperative serum follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels were extremely high, and the serum testosterone level was extremely low. In the twin with two testes, preoperative serum FSH, LH, and testosterone levels were normal. After successful transplantation of a testis from the twin with two testes to the twin with no testes, using the microvascular technique, the recipient twin developed a normal serum testosterone level within 2 hours of surgery; his FSH and LH levels came down toward a normal range more slowly over the ensuing 4 weeks. The donor's FSH level became mildly elevated 2 days postoperatively but returned to normal by 3 months. Thereafter, serum FSH, LH, and testosterone levels remained persistently normal in both twins. In the donor, pre- and postoperative sperm counts were normal. Preoperatively the recipient's semen had no sperm, but postoperatively the sperm content has slowly increased to normal levels.