The high prevalence of human immunodeficiency virus (HIV-1) infection in populations at risk in Miami prompted a seroepidemiologic study of both HIV-1 and the human T-cell leukemia virus type I (HTLV-I), a closely related virus, in our patients receiving chronic hemodialysis. One hundred twenty-nine patients undergoing hemodialysis in 1986 and 1987 were tested for antibody against both viral antigens by EIA (Abbott Laboratories, Abbott Park, IL). Seroreactive samples for HIV-1 and/or HTLV-I were confirmed by Western blot and, for HTLV-I, by viral cultures. Thirty patients (23.2%) were positive for retroviral infection (22 for HIV-1 alone, four for HTLV-I alone, and four for both HIV-1 and HTLV-I). The most important risk factor was intravenous drug use, followed by blood transfusion. Patients with HIV-1 had lower T4-T8 ratios and higher mortality than those with HTLV-I infection alone. It was concluded that HTLV-I, as well as HIV-1, infection is endemic in chronic dialysis centers in Miami. The clinical consequences of HTLV-I infection in relatively immunocompromised patients with chronic uremia who are undergoing chronic hemodialysis remains to be established.