A series of 100 cases is presented : 34 female and 66 male pseudohermaphrodites (MPH). Plasma levels of testosterone (T) and its precursors have been made in basal conditions and in the MPHs after hCG stimulation test. In spite of detailed hormonal, anatomical and chromosomal studies no etiological diagnosis was made in about half the MPHs. In 45% of those, the response to hCG was inadequate suggesting that ambiguous development of the genital ducts, urogenital sinus and external genitalia is associated with a wide spectrum of defective testicular gonadogenesis. The endocrine function of these dysgenetic testes may worsen with age. Determination of T and several of its precursors is essential for the correct diagnosis of inborn errors in testicular biosynthesis which was made thrice by systematic study of so-called simple hypospadias. It is suggested that systematic hormonal study should be made in all children, and preferably newborn, with ambiguous genitalia associated with micropenis or bilateral cryptorchidism.