Antibodies reactive with human T-cell leukemia virus type I (HTLV-I) proteins p19, p24, gp46, p56, and gp68 were detected in four of 27 patients with mycosis fungoides/Sézary syndrome (MF/SS) and one patient with Kaposi's sarcoma using radioimmunoprecipitation and Western blot analysis. Seroreactivity patterns to HTLV-I proteins of MF/SS sera were indeterminate or limited in comparison with sera of patients with adult T-cell leukemia/lymphoma. HTLV-I gag- and tax/rex-specific DNA was demonstrated in peripheral blood from three of the MF/SS patients and from the patient with Kaposi's sarcoma by the polymerase chain reaction. HTLV-I-specific DNA sequences were not detected in a cohort of seven seronegative MF/SS patients. The frequency of HTLV-I infection was four of 27 or 14.8% among the MF/SS patients, which is several hundredfold higher than in normal blood donors. The present data suggest a possible association of HTLV-I or a related retrovirus with mycosis fungoides/Sézary syndrome and Kaposi's sarcoma.