The right testis and epididymis were removed at necropsy from 31 patients who had died with AIDS and examined microscopically. The testes showed interstitial fibrosis in 22 cases, thickening of seminiferous tubule basement membranes in an overlapping 21, and varying degrees of maturation impairment in all. Arrest of spermatogenesis usually occurred at the spermatocyte or spermatid stage. Five epididymides showed an obstructive picture, two with an associated epididymal cytomegalovirus infection. We discuss the possible pathogenesis of the changes in the HIV-infected male genital system and briefly comment on the clinical implications of our findings.