We previously reported a high incidence of acquired immune deficiency syndrome (AIDS) in Kinshasa, Zaire, as well as a high frequency of antibody to human immunodeficiency virus (HIV), which includes HTLV-III and LAV viruses, in persons without AIDS.1,2 In this report we assessed the frequency of HIV virus infection in persons with and without clinical AIDS and the association of virus isolation to presence of antibody. We isolated HIV from 27 (77%) of 35 patients with AIDS, and 5 of 9 patients with AIDS-related complex (ARC). Virus was also isolated from plasma and cerebrospinal fluid of patients in the study. The presence of antibody was a reliable marker for virus infection in African patients with AIDS. HIV was isolated from 5 of 27 control patients without AIDS, 3 of whom had normal T helper to T suppressor ratios and normal numbers of T helper cells. Two of these patients had no detectable antibody to HIV by ELISA or Western blot methods. In a population, such as the general heterosexual population of Kinshasa, with frequent infection by HIV and with few clearly definable risk groups, screening for antibodies to HIV may not be sufficient to identify some virus infected persons.