Pathogenesis of precocious puberty in patients with pineal tumor and ectopic pinealoma was studied immunohistochemically, focusing on the endocrine function of the tumors and on the correlation between the tumor secreting hormone and Leydig cells. Optic chiasmal germinomas secrete β-subunit-like FSH and LH, while choriocarcinoma in the pineal region secretes hCG (α, β). Precocious puberty is recognized only in the latter case. On the other hand, endogenous gonadotropins are localized in immature Leydig cells of the fetus, infant and adult testes. The number of hCG positive immature Leydig cells increased 3 hours after hCG injection. Thus it became obvious that not only endogenous LH (hCG) but also exogenous LH (hCG) bind to immature Leydig cells; that is, Leydig cells exist as target cells of the gonadotropin. These Leydig cells synthesize steroid hormones such as androgen, and the hormones give the sexuality of a male to a boy. As a result, it can be concluded that precocious puberty is induced by the tumor secreting gonadotropin.