We conducted a survey using serology and polymerase chain reaction assays to detect HIV 1 and/or HTLV-I antibodies and viral DNA, respectively, in 113 intravenous drug abusers and in 62 sexually active high risk individuals attending two Drug Addict Centres and a Centre for Venereal Diseases in Buenos Aires, Argentina. At the time of the survey 137 of these subjects were known to be HIV 1 seropositive but none of them was symptomatic. Serological tests for HTLV-I were found to be positive in 38 (21.7%) of the HIV 1 positive individuals, whereas all of the 38 HIV 1 seronegative subjects were also seronegative for HTLV-I antibodies. Gene amplification assays carried out in blood sample DNA from the 38 HIV 1/HTLV-I seronegative individuals, revealed HIV 1 DNA in six out of 28 intravenous drug abusers (21.5%). One subject (2.6%) was positive for both HIV 1 and HTLV-I DNA sequences, whereas four (10.5%) showed HTLV-I DNA only. To determine whether these individuals were infected with HTLV-I and/or HTLV-II, DNA samples were also amplified with HTLV-II specific primers and no evidence of HTLV-II infection was observed. None of the subjects seroconverted according to conventional serological tests during the 2 year follow-up period. The 10 seronegative subjects belonging to the sexual risk group were negative for both HIV 1 and HTLV-I in polymerase chain reaction assays. We conclude that not only HIV 1, but also HTLV-I is a widely spread infection in intravenous drug abusers and sexually active high risk individuals in Argentina.(ABSTRACT TRUNCATED AT 250 WORDS)