We demonstrate that antitestosterone autoantibody is produced in a 24-yr-old woman with hypergonadotropic hypogonadism. The serum testosterone level measured by RIA was extremely elevated (5.80 ng/mL); after elution, serum testosterone had returned to a normal female value (0.21 ng/mL). The clinical features were suggestive of no androgen activity. Primary follicles were present in the patent's ovary. After gonadotropin treatment, conception was achieved, and a normal female infant was delivered. A gel filtration study showed that the testosterone-binding activity was eluted at the position of 15,000-kDa Ig. Scatchard analysis revealed a low affinity antibody; the association constant was 0.034 x 10(3) mol-1, and the maximal binding capacity was 162 mumol/mL. An immunoprecipitation study the chain-specific antibodies showed that the antitestosterone autoantibody belonged to kappa-type IgG. This subject is the first reported case with an endocrine disorder who possessed autoantibodies against testosterone.