Persons infected with HIV show changes in all aspects of immunity. Cellular immunity is affected most markedly, with decreased numbers of CD4+ (helper) and CD8+ (suppressor) T cells. Cellular immune function in vitro is frequently abnormal, as assessed by ability of T cells to proliferate in response to antigens or mitogens. Impaired humoral immunity is manifested in polyclonal B-cell activation and in impaired antibody responses to soluble antigens in vivo and in vitro. Numerous functional defects of macrophages have also been reported. The most valuable clinical laboratory tests for evaluation of the immune status of the HIV-infected individual are enumeration of circulating CD4+ and CD8+ cells and quantitation of circulating immunoglobulins. The results of these tests can help differentiate the immunosuppression caused by HIV from other causes of immunodeficiency. Helper T-cell counts are also important for selecting appropriate treatment regimens. Combinations of the results of these tests with those of virus culture or antigen detection assays provide additional prognostic information. Future research may help us recognize other patterns of immunodeficiency with specific clinical implications. It is hoped that new therapies, both antiviral and immune reconstitutive, will soon become available; at such time immunologic testing will be able to chart the return to normal immune function.