A large cross-sectional serologic survey for human T cell leukemia virus type 1 (HTLV-1) antibody was conducted in 3,177 Ivory Coast residents to evaluate the prevalence of HTLV-1 and to determine possible risk factors and correlates of HTLV-1 infection. Of the 3,177 serum samples, 110 (3.5%) were positive for antibody to HTLV-1 by indirect immunofluorescence assay and Western blot. The prevalence of HTLV-1 antibody in the general adult population was 1.8% and increased significantly with age. No difference between males (1.5%) and females (2%) was found. The highest prevalences were observed in female prostitutes (7.4%), patients with neurologic syndromes (5.8%), and lepers (13.7%). The high prevalence of HTLV-1 infection in prostitutes suggests that heterosexual contact is involved in the transmission of HTLV-1 and that prostitutes could play an important role in the spread of the virus in Africa. The high prevalence of HTLV-1 in patients with neurologic syndromes confirms the association between HTLV-1 and some type of neuropathies, as has been observed in the West Indies and Japan. The high prevalence observed in lepers deserves further investigation to find the cause of the association. Twenty-five individuals, including prostitutes, were coinfected with HTLV-1 and human immunodeficiency virus (HIV). Prospective studies are necessary to evaluate the exact role of HTLV-1 alone or in combination with HIV in inducing specific diseases.