HIV infection induces both immune deficiency and immune stimulation. Central to the pathology of HIV infection is reduction in the numbers and function of CD4 T cells. Impaired functions include decreased proliferation, IL-2 receptor expression and production of lymphokines (IL-2 and gamma interferon (IFN]. HIV infection stimulates B cells and CD8 T cells. This is seen relatively soon after HIV infection. Increased activation and immaturity are seen in both these cell groups. In vitro studies confirm HIV stimulation of these cells. Studies have been conducted on patients with AIDS and opportunistic infection (OI) or Kaposi's sarcoma (KS), with AIDS-related complex (ARC) or with persistent generalized lymphadenopathy (PGL), as well as on asymptomatic HIV-seropositive and -seronegative homosexually active men. The latter group has been followed at 6-month intervals for the past 2-3 years. Those who seroconverted (became HIV-infected) were studied to investigate early changes following HIV infection. To delineate the immunopathology of infection with HIV, serial testing of seropositive individuals was carried out to determine the rate of CD4-T-cell reduction. Lowered CD4-T-cell number and percentage and CD4/CD8 ratio correlate with the occurrence of AIDS and with survival after AIDS-KS diagnosis. Seropositive individuals, however, differed markedly in the rate of CD4-T-cell reduction; in some, no reduction in CD4 cells occurred over a two-year period of observation. We propose that, in individuals in which CD4 levels have reached a plateau, effective host resistance to further CD4 cytoreduction has occurred.