Human T-cell lymphotropic virus type I (HTLV-I) infection is endemic in southwestern Japan, the Caribbean basin, Colombia, Africa and in several isolated populations in Papua New Guinea, the Solomon Islands and Vanuatu. To determine the seroprevalence of HTLV-I infection in Singapore, we tested sera from 115 hospitalized patients with acute nephritis, 50 patients with suspected leptospirosis, 34 patients with non-A, non-B hepatitis, and from 28 healthy volunteers for IgG antibodies against HTLV-I using an enzyme-linked immunosorbent assay. Antibodies were detected in sera from 6 of the 199 patients and from 3 of the 28 healthy volunteers, but these positives could not be confirmed by Western immunoblotting. Our data are consistent with other reports of low seroprevalence of HTLV-I infection despite extensive Japanese contact in Korea, Taiwan, the People's Republic of China and Micronesia. Further studies on a larger sample size, however, are necessary to confirm the absence of any focus of infection in the Singapore population.