The immunohistochemical reaction to oncostatin M (OSM) was studied in normal human testes at different ages (fetuses, newborns, children, pubertal boys, adults, and elderly men), as well as in several testicular disorders including carcinoma-in-situ cells (CIS), germ cell tumors, benign functioning Leydig cell tumor, androgen insensitivity syndrome, Klinefelter's syndrome, and cryptorchidism. Positive OSM immunostained Sertoli cells were only observed in fetuses. In normal testes, intense OSM immunoreaction was found in the Leydig cells of fetuses, newborns, and adults. Leydig cell immunoreaction was weak in elderly men and absent in children and pubertal boys. In some testicular disorders (Leydig cell tumor, cryptorchidism, and CIS), Leydig cell immunoreaction was as intense as in normal adult testes. This immunoreaction was heterogeneous in androgen insensitivity syndrome and was absent in Klinefelter's syndrome and intratubular seminoma. No recognizable Leydig cells were observed in the other testicular tumors. The findings of our study suggest that, in humans, the down-regulation of OSM immunoexpression in Sertoli cells occurs early, and that OSM immunoreaction in the Leydig cells is associated with functionally active and differentiated Leydig cells.