Morphologic and steroid biochemical studies were performed on surgical specimens from a 6-yr-old boy with a Leydig cell tumor. Active spermatogenesis to early spermatid stage was found in the testicular tissue of the tumorbearing testis, and absence of spermatogenesis was observed in a biopsy of the contralateral testis. Tumor tissue actively metabolized tritiated progesterone, pregnenolone and cholesterol to testosterone and androstenedione.1 Correlation of these data with plasma androgen levels from this patient's peripheral and spermatic veins led the authors to conclude that, as in lower species, high local levels of androgen will initiate spermatogenesis in man.