The acquired immunodeficiency syndrome (AIDS) is epidemic in Central Africa. To determine the prevalence of AIDS virus infection in East Africa, we studied 90 female prostitutes, 40 men treated at a clinic for sexually transmitted diseases, and 42 medical personnel in Nairobi, Kenya. Antibody to human T-cell lymphotropic virus Type III (HTLV-III) was detected in the serum of 66 percent of prostitutes of low socioeconomic status, 31 percent of prostitutes of higher socioeconomic status, 8 percent of the clinic patients, and 2 percent of the medical personnel. The presence of the antibody was associated with both immunologic and clinical abnormalities. The mean T-cell helper/suppressor ratio was 0.92 in seropositive prostitutes and 1.82 in seronegative prostitutes (P less than 0.0001). Generalized lymphadenopathy was present in 54 percent of seropositive prostitutes and 10 percent of seronegative prostitutes (P less than 0.0001). No constitutional symptoms, opportunistic infections, or cases of Kaposi's sarcoma were present. Our results indicate that the epidemic of AIDS virus infection has, unfortunately, spread extensively among urban prostitutes in Nairobi, Kenya. Sexual exposure to men from Central Africa was significantly associated with HTLV-III antibody among prostitutes, suggesting transcontinental spread of the epidemic.