Patients attending sexually transmitted disease (STD) clinics in Baltimore (n = 4880) and New Orleans (n = 1054) were surveyed in 1987 to estimate the prevalence of human T lymphotropic virus (HTLV)-I/II infection. In Baltimore, 0.4% (95% confidence interval [CI], 0.2-1.1) were HTLV-I/II-seropositive and 4.9% were human immunodeficiency virus (HIV-1)-positive. In New Orleans, 1.8% (CI, 1.2-2.9) of sera were HTLV-I/II-seropositive and 5.1% were HIV-1-seropositive. In both cities, HTLV-I/II prevalence increased significantly with age, and the New Orleans age- and sex-adjusted HTLV-I/II prevalence was significantly higher than that of Baltimore (P less than .001). In Baltimore, almost all HTLV-I/II seropositivity was associated with a history of parenteral drug use or sexual contact with partners who were drug users or male homosexuals. In addition, individuals in both cities who were seropositive for HIV-1 or syphilis were significantly more likely to be HTLV-I/II-seropositive.