Biopsies of testicular specimens taken from 41 patients that were diagnosed as having idiopathic Sertoli-cell-only syndrome were classified into two types, A and B, on the basis of histological and immunohistochemical findings. Thirty eight specimens that were classified as type A exhibited seminiferous tubules of small diameter and with tubular wall hyalinization, but containing normal adult type Sertoli cells. The other three specimens that were classified as type B showed no seminiferous tubular wall hyalinization, and their Sertoli cells had vimentin distribution localized in the subnuclear cytoplasm and had a pseudostratified lining, features resembling the appearance of fetal Sertoli cells. In one patient with a seminoma, a comparative study of the same testis prior to and post-irradiation was undertaken. Judging from this, postpubertal depletion of the germ cell population was considered to be responsible for the tubular atrophy observed in type A. Type B testes, though small in number, were characterized by a morphology distinct from the type A, but their pathogenesis remains unknown.