The seroprevalence of HTLV-I/II was evaluated in 1247 Italian individuals at high risk for HIV infection. The population studied consisted of 985 intravenous drug users (IVDUs), 474 of whom on methadone maintenance and 511 in a therapeutic community, 110 HIV-infected patients in various stages of HIV-related disease and 152 hemophiliacs. Sera were screened for antibody to HTLV-I/II by enzyme immunoassay (EIA) and confirmed by Western blot and radioimmunoprecipitation assay. Confirmed positive samples were further differentiated by EIA using HTLV-I and HTLV-II specific peptides. The overall prevalence of anti-HTLV-I/II was 4.0% in IVDUs, with the highest prevalence (8.2%) among HIV-infected symptomatic patients. None of the hemophiliacs was anti-HTLV-I/II positive, even though 63.1% tested positive for HIV antibodies. The trend of seroprevalence in drug users and the evaluation of possible risk factors demonstrated that HTLV-I/II infection has been present in Italy before the onset of HIV epidemic. The overall seroprevalence showed no significant changes during the 10 year period covered by this survey but correlated with HIV seropositivity, age and duration of drug use. Peptide testing showed that HTLV infection was mainly due to HTLV-II.