We studied a cohort of 45 homosexual men with the acquired immunodeficiency syndrome, 78 with persistent unexplained generalized lymphadenopathy, and 160 asymptomatic homosexual controls for serologic evidence of infection with human T-lymphotropic virus type III (HTLV-III). Study participants were recruited from a community-based health center and a university hospital practice. Ninety-eight percent of men with the syndrome and greater than 90% of men with generalized lymphadenopathy had antibody to HTLV-III, while 21% of the controls were positive (p less than 0.001). Six patients with generalized lymphadenopathy developed the acquired immunodeficiency syndrome over 2 years; all were seropositive for HTLV-III. Thirty-six asymptomatic controls had had sexual contact with a man with the syndrome; receptive anal intercourse in this group was associated with seropositivity for HTLV-III. These data suggest that persistent generalized lymphadenopathy and the acquired immunodeficiency syndrome are part of a clinical spectrum of HTLV-III infection and that most high-risk homosexual men in some regions of the United States have not yet been infected with this virus.