PIP: To learn more about the relative prevalence of viruses from the human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) groups in Ghana, serum samples were collected in 1986 from 47 men and women with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex (ARC), 57 apparently healthy individuals, and 11 AIDS-free hospital inpatients. Western blot analysis revealed a total of 46 reactive sera. 43 of the 47 serum samples from those with AIDS or ARC were positive; 6 were seropositive for HIV-1, 18 for HIV-2, 17 for both HIV-1 and HIV-2, and the remaining 2 were not reactive with glycoproteins. Of the 2/10 individuals with mild symptoms of HIV infection who proved to be seropositive, 1 was positive for HIV-2 and 1 for HIV-1 and HIV-2. There were no seropositive reactions among the AIDS-free hospital patients, and only 1 such reaction (seropositive for HIV-2) among the healthy individuals. These findings indicate that both HIV-1 (6 cases in this series) and HIV-2 (20 cases) are responsible for the development of AIDS in Ghana, and that there is a high prevalence (18 cases) of cross-reactivity between the 2 viruses. There was no evidence of SIV infection. Further research is needed to determine whether these findings are a result of cross-reactivity between envelope proteins of HIV-1 and HIV-2, double infection of AIDS patients, or infection with a new variant strain. Since prostitutes comprised 25 of the 47 AIDS/ARC patients and 6 of the 10 with mild symptoms of HIV infection, they are an important target for preventive efforts.