This study investigated HTLV-II infections in a cohort of HIV-I-infected intravenous drug users (IVDAs)(n=103)in Dublin. Serological and molecular analyses have demonstrated that 15 (14.6%) were infected with HTLV-II, which is the highest rate of infection so far documented in Europe, and is equivalent to that in urban areas of the United States. Restriction fragment length polymorphism (RFLP) analysis of the env region of the provirus revealed only HTLV-IIa infection. There was no evidence of HTLV-IIb subtype infection. Phlyogenetic analysis of the nucleotide sequences of the long terminal repeat (LTR) confirmed infection with the HTLVE-IIa subtype, and the demonstrated that the viruses clustered closely with HTLV-IIa isolates from North American IVDAs. Previous observations that IVDAs in southern Europe appear to be infected predominantly with the HTLV-IIb sub-type, along with the present report and evidence that the IVDAs in Sweden are infected with HTLV-IIa subtype, suggest different origins of HTLV-II infection in Europe.