In 1990/1991, 885 prostitutes residing in 11 of the 12 Local Government Areas (LGAs) of Lagos State, Nigeria, participated in a cross-sectional study to determine current seroprevalence of antibodies to human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2), and human T-cell lymphotropic virus type I (HTLV-I). The overall prevalence of HIV-1 was 12.3%, of HIV-2, 2.1%, and of HTLV-I, 2.8%. HIV-1 seropositivity did not vary significantly by age, socioeconomic class, or nationality, but HIV-1 seroprevalence was significantly elevated for prostitutes resident in the Port area of Lagos which serves as a crossroads for international and national commerce (OR = 2.3; 95% CI = 1.1, 4.6). HIV-2 infection was significantly associated with low socioeconomic class (OR = 3.7; 95% CI = 1.2, 10.8) and non-Nigerian nationality (OR = 6.7; 95% CI = 2.5, 18.4). Prevalence of HTLV-I infection increased significantly with age (OR = 2.3; 95% CI = 1.0, 5.3). The high seroprevalence of HIV-1 in this survey, compared with previous surveys reported in the last several years and the correlation between high prevalence and areas of international commerce suggest that HIV-1 is spreading in this area of Nigeria. Intensified prevention campaigns are needed to address this possible emerging epidemic.