Immune complex assays (and other immunologic tests) were performed on sera from 162 patients with the acquired immunodeficiency syndrome (AIDS) and 275 AIDS-related subjects. Immune complexes were detected in 89% of AIDS patients and 93% of homosexual men with lymphadenopathy. Immune complex levels in AIDS patients were not associated with a particular risk group or with types of opportunistic infection or malignancy; however, they correlated with other laboratory features of the immune defect (depression in T helper cells and T helper/suppressor-cell ratio, and IgG levels). Immune complexes were also detected in a lesser proportion of risk-group controls (homosexual men, hemophiliacs, Haitians). In risk-group controls, immune complex levels were associated with certain features reflecting sexual practice, blood product exposure, or infection, but these features did not account for the higher levels found in AIDS patients. In appropriate situations, immune complex assays may be of value as screening tests or, possibly, as prognostic indicators for AIDS or AIDS-related syndromes.