Report of the case of a 21-year-old man who presented with gynecomastia and was found to have a testicular embryonal carcinoma producing human chorionic gonadotropin (HCG), associated with massive hyperplasia of Leydig cells. While the morphologic features of Leydig cell hyperplasia have been documented with excess endogenous luteinizing hormone (LH), as in Klinefelter's syndrome, and following administration of exogenous HCG, to our knowledge, this is the first description of the histologic and ultrastructural changes in Leydig cells associated with increased secretion of endogenous HCG. The Leydig cell stimulation which results from excess endogenous HCG is unusual in that it causes an elevation of estrogens which may be greater than the rise in androgens. It is concluded that embryonal cell carcinoma of the testis, producing HCG, may lead to Leydig cell hyperplasia with morphologic features similar to those seen in other conditions with such hyperplasia, and that there is no correlation between those morphologic features and testosterone production.