The testes and prostates of 14 patients with acquired immune deficiency syndrome (AIDS) for whom autopsies were performed were examined for the presence of human immunodeficiency virus (HIV) and the pathologic alterations seen in AIDS. Histologically, the testes contained peritubular fibrosis and variable spermatogenic arrest, which were inconsistent with the young age of these patients. There were also numerous foci of germ cell degeneration and occasional germ cell loss. The Leydig cells were atrophic and decreased in number. The prostates contained increased numbers of concretions. Sections of testis and prostate were stained with an anti-HIV P17 monoclonal antibody with the use of the avidin-biotin technique. Small scattered foci of positive staining were identified in 8 of 14 testes (57%). They were located over one or several degenerating germ cells and the surrounding Sertoli cells. In addition, in 9 of 14 prostates (64%) there were a few minute foci of positive staining in several adjacent glandular epithelial cells. In one case the testis was positive and the prostate was negative, whereas in two cases the testes were negative and the prostate positive. In contrast, 22 testes and 22 prostates of control non-AIDS patients, read double-blind, were negative. The positive controls were HIV-infected tissue culture HUT 78 lymphoma cells in which there were many scattered positive cells. The results indicate the presence of focal HIV-associated protein in the testes and prostates of patients with AIDS, particularly within the foci of germ cell degeneration. The present observations are in accordance with previous research demonstrating the presence of HIV in the seminal fluid of patients with AIDS and appears to indicate the presence of an infection of the male genital tract by the HIV virus.