The pathogenesis of the increased incidence of subfertility in cryptorchidism remains controversial, and the concept that the prepubertal testis is a quiescent organ continues to be held by some. Histomorphometric analyses including total and differential germ cell counts and Leydig counts were performed on semithin sections of biopsies from 459 unilateral undescended testes and 356 of their contralateral descended partners in order to further investigate these two issues. The results demonstrate reduced numbers of Leydig cells, delayed disappearance of gonocytes, delayed appearance of adult dark spermatogonia, failure of primary spermatocytes to appear, and reduced total germ cell counts in the undescended testis. These findings support the hypothesis that understimulation of the testis leads to reduced numbers of Leydig cells which leads to delayed and defective maturation of germ cells and reduced total germ cell counts which underlie the increased risk of subfertility in cryptorchidism. The appearance of adult dark spermatogonia in the first year of life and the onset of meiosis at three years of life in the contralateral descended testes contradicts the view that the prepubertal testis is quiescent. Complete evaluation of prepubertal testicular biopsies should include total and differential germ cell counts and Leydig cell counts.