Mechanisms underlying human immunodeficiency virus type 1 (HIV-1) infection of the male reproductive tract and the sexual transmission of HIV-1 through semen are poorly understood. To address these issues, the authors performed morphologic and immunocytochemical analyses of reproductive tissues obtained at autopsy from 43 male acquired immune deficiency syndrome (AIDS) patients. Monoclonal antibodies recognizing different subpopulations of white blood cells were used to detect leukocyte infiltration and map the location of potential lymphocytic/monocytic HIV-1 host cells and immunocytochemistry and in situ hybridization techniques were used to detect HIV-1-infected cells in the testis, excurrent ducts, and prostate. Distinct pathologic changes were observed in a majority of testes of AIDS patients that included azoospermia, hyalinization of the boundary wall of seminiferous tubules, and lymphocytic infiltration of the interstitium. The reproductive excurrent ducts and prostate appeared morphologically normal except for the presence of focal accumulations of white blood cells in the connective tissue stroma. In the testis many white blood cells were shown to be CD4+, indicating the presence of abundant host cells (T-helper/inducer lymphocytes and macrophages) for HIV-1. Furthermore macrophages and cells of lymphocytic morphology were observed migrating across the boundary walls of hyalinized seminiferous in tubules to enter the lumen. In 9 of the 23 cases tested for HIV-1 protein expression by immunocytochemistry. HIV-1 + cells of lymphocytic/monocytic morphology were found in the seminiferous tubules and interstitium of the testis, epididymal epithelium, and connective tissue of the epididymis and prostate. One patient with epididymal blockage had accumulations of HIV-1-antigen-positive cells of macrophages morphology in the distended lumen of the efferent ducts. There was no evidence of active HIV-1 infection in germ cells or Sertoli cells of the seminiferous tubules or other epithelial cells lining the excurrent ducts or prostatic glands.