Twenty-seven histologically confirmed Kaposi sarcoma (KS) patients resident in the Kivu Lake area of eastern Zaire were examined for immune competence. Only KS cases of the endemic African type have been observed in this high-incidence area. The median duration of the symptoms was 6 years and ranged from 1 to 38 years. Forty-one controls matched for age, sex and tribe and unrelated to the KS patients were selected from the community. Thirteen additional controls were first-degree relatives of the KS patients. No evidence of immune suppression among KS patients was found and there were no significant differences in the immune status between KS patients and controls. Total lymphocytes, B and T cells, and OKT4 + and OKT8 + cells varied within the normal range. Grouping of the KS patients in categories according to duration and disease extent did not reveal significant differences in their immune status. The number of KS patients reacting positively in a skin test to 5 recall antigens and I mitogen was similar that of controls, except in the case of candidin, to which a higher number of KS patients were negative. The serum levels of immunoglobulins, complement factors and circulating immune complexes were comparable in KS patients and controls. Indicators of inflammatory processes [white blood cells (WBC), complement-reactive protein (CRP)] were positive in 27% of the KS patients. The prevalence and mean titer of antibody against cytomegalovirus (CMV), Epstein-Barr virus (EBV), hepatitis B virus (HBV) and syphilis were similar in KS patients and in controls.