In two groups of subjects at risk for acquired immune deficiency syndrome (AIDS), homosexual males and intravenous drug users with persistent generalized lymphadenopathy, humoral and cell-mediated immunity were compared. A small group of patients with definite AIDS were also studied. It was found that levels of immunoglobulins, serological markers for virus and other infections, cell-mediated immunity and histology of lymph nodes were similar in homosexuals and drug users, whereas the lymphocyte sub-populations differed completely. The number of T4+ lymphocytes was markedly decreased in homosexuals but normal in drug users; the number of T8+ lymphocytes was much higher in drug users than in homosexuals. This discrepancy may explain the extremely low prevalence of AIDS cases observed among Swiss drug users as compared with the high frequency noted among homosexuals.