Abstract.We treated an 11-year-old boy with a testicular Leydig cell tumor. We analyzed the testosterone production of this tumor by immunolocalization of steroidogenic enzymes and in vitro three-dimensional histoculture. Spermatic venous blood from the tumor bearing testis had noticeably high concentrations of testosterone and androstenedione. The tumor had the characteristic ultrastructural features of steroid producing cells and was immunoreactive for P450scc (side chain cleavage), 3I3HSD (hydroxysteroid dehydrogenase) and P450c17 (17a-hydroxylase).Three-dimensional collagengel-supported histoculture demonstrated that the tumor tissue in the culture maintained its histologic architecture, expression of steroidogenic enzymes, and secretion of testosterone into the medium for up to 7 days in culture. Histoculture preserved in vitro testosterone production in this case of testicular Leydig cell tumor.Key words: Testis, Leydig cell tumor, Histoculture, Testosterone, Immunohistochemistry (Endocrine Journal 43: 73-78, 1996) LEYDIG cell tumors of the testis are rare but the majority are associated with various steroid biosynthetic abnormalities including testosterone and estrogen overproduction [1][2][3][4][5]. Clinical hormonal analysis of testicular Leydig cell tumor has been extensively performed [1][2][3][4][5], but no detailed analysis of steroidogenesis of a Leydig cell tumor itself including in vitro studies has been reported. We had the opportunity to examine an 11-yearold boy with a Leydig cell tumor of the testis with increased plasma androstenedione and testosterone concentrations and pseudoprecocious puberty. Pathological analysis of the resected specimens including electron microscopy and immuno- The patient was an 11-year-old Japanese boy. His parents noticed linear and skeletal growth acceleration and virilization at the age of 9 years, but he did not receive medical attention at that time. He presented to the pediatrician with complaints of testicular enlargement and premature puberty at the age of 11 years and was referred to Tohoku University Hospital, Sendai, Japan for evaluation of a testicular mass.His height was 160.05 cm and his weight was 58.0 kg on admission, both greater than the 95