High-risk sexual behavior as risk factor for human T-cell lymphotropic virus type I (HTLV-I) infection was assessed in cross-sectional studies with 1,183 prostitutes and 1,166 pregnant women in Kinshasa, Zaire. Eighty six (7.3%) prostitutes were positive for HTLV-I. The seroprevalence among prostitutes from the regions along the equator was 12.7%, whereas among prostitutes from the other regions it ranged between 0 and 4.3%. In the prostitutes from the high-prevalence regions, but not in the prostitutes from the low-prevalence regions, HTLV-I infection was associated with increasing age [odds ratio (OR) = 1.1 per year increment], active syphilis (OR = 2.3), and human immunodeficiency virus (HIV) infection (OR = 2.0). Forty three (3.7%) pregnant women were HTLV-I seropositive. Among the women from low-prevalence regions, there was no significant difference in HTLV-I seroprevalence between prostitutes (4.3%) and pregnant women (3.5%). In a group of 409 prostitutes who were observed for a mean duration of 23 months, the incidence of HTLV-I infection was 0.7 per 100 women-years, whereas the incidence of HIV infection was 9.8 per 100 women-years. We conclude that in Kinshasa prostitution per se was not associated with an increased risk of HTLV-I infection.