Homosexual men were studied for associations among human T-lymphotropic virus type III (HTLV-III) infection, Epstein-Barr virus (EBV) infection, and T cell abnormalities. The presence of IgG antibody to EBV capsid antigen and antibody to EBV early antigen was significantly associated with augmented counts of suppressor T cells in healthy HTLV-III-seronegative men. HTLV-III-seropositive asymptomatic subjects had significantly enhanced titers of antibody to EBV and lower ratios of helper to suppressor T cells compared with HTLV-III-seronegative homosexual men. Of three men who seroconverted to HTLV-III, two had a greater than fourfold increase in titer of IgG antibody to EBV capsid antigen after seroconversion. These results suggest that the interaction of HTLV-III and EBV and their immunologic perturbations are significant in the natural history of this retrovirus infection in homosexual men.