A partial defect in testosterone (T) secretion due to a 17 beta-hydroxysteroid dehydrogenase deficiency has been documented in a patient with advanced metastatic interstitial cell carcinoma of the testis. Basal serum T was decreased (83 ng/ml), whereas androstenedione was markedly elevated (847 ng/ml). Dehydroepiandrosterone sulfate and estrone were moderately increased, while LH, FSH, and estradiol were minimally increased. After hCG administration, androstenedione rose to much greater degree than T. Fluoxymesterone failed to suppress circulating androgens and estrogens to below basal values, although LH and FSH were suppressed to immeasurable levels. These data support a partial enzyme deficiency within the tumor. They also suggest that although the tumor was autonomous, it still was capable of being stimulated by exogenous gonadotropins.