The present investigation documents morphological characteristics of human Sertoli cells of aged males. Testicular material was obtained from 35 patients (age 62-84 years) with carcinoma of the prostate who had received no previous anticancer therapy. As revealed by light and electron microscopy the appearance of the germinal epithelium showed great individual variations. In all cases examined, however, the occurrence of multinucleate Sertoli cells was a common finding. In seminiferous tubules with intact spermatogenesis these cells closely resembled the normally occurring variants, whereas they displayed features reminiscent of immaturity in the absence of germ cells. It is hypothesized that the nuclei of Sertoli cells in the special situation of aging may resume the capacity to divide, an ability normally restricted to immature cells. Thus, mitosis without subsequent cytokinesis might be an explanation for the formation of multinucleate Sertoli cells.