The serological and immunological parameters, disease patterns, and social characteristics of 39 human immunodeficiency virus type 2 (HIV-2) seropositive CDCIV cases seen in Dakar, Senegal were studied. These data were compared with those obtained from 48 HIV-1 seropositive CDC stage IV patients. Social characteristics of populations infected with HIV-1 or HIV-2 were clearly different. A patient sex ratio of three men to one woman was found for both viruses. In addition, the immune status of nonsymptomatic HIV-1 and HIV-2 seropositive people was evaluated. The correlation between abnormalities of the immune system and clinical status was similar for the two infections. Clinical symptoms of both diseases were the same, but this cross-sectional study could not address the questions of differences between the two infections in latency and development of disease or specific manifestations of HIV-2 infection. This study suggests that HIV-2 infection may contribute to the present AIDS epidemic in West Africa.