In this report, the results of seroepidemiologic studies of human T-lymphotropic virus type I (HTLV-I) and type II (HTLV-II) infections in different population groups in Argentina have been compiled. The studies have shown a high prevalence of HTLV-I/II infection in blood donors in the provinces in the north of Argentina (1.0% in Jujuy, 0.7% in Salta, and 0.6% in Formosa) and a low prevalence in the provinces in the central region of the country (<or=0.1%). High rates of HTLV-I (0.45%-2.78%) and HTLV-II (2.78%-21.9%) infections have been documented in Native Indian groups and have highlighted the importance of sexual and mother-to-child transmission of the viruses. The presence of HTLV-I/II antibodies in HIV-infected individuals indicates that both viruses are circulating in certain high-risk population groups. HTLV-I/II was detected in intravenous drug users (2.56%-21.7%), homosexual men (0.5-1.33%), prostitutes (0.8%), and hemophiliacs (2.8%). In conclusion, HTLV-I/II infection is circulating in low- and high-risk populations in Argentina, and these findings would support the view that selective screening for HTLV-I/II should be implemented to reduce the spread of infection.