In Taiwan, the epidemiological status of HCV infection is similar to those observed in other areas of the world, with 1.0% prevalence among adult volunteer blood donors and high prevalences among the high risk groups, by the detection of anti-HCV with synthetic peptide antigens. However, unusually high prevalences, 35.1%, 15.8% and 14.2%, were observed among adult populations in three of the five aboriginal communities. No difference in sex specificity was noted. In 37 (75.0%) of the 48 anti-HCV-positive cases, HCV-RNA was detected by reverse transcription polymerase chain reaction (RT-PCR) assay. None of such particular risk factors as tattooing, sexual promiscuity, operation, blood transfusion, nor intravenous drug abuse could be accounted for this high prevalence of HCV infection. No helpful supporting evidence for ethnic specificity was noted, either. Although a possible sexual transmission between spouses was observed, it is unlikely to be the main cause of the high prevalence in these aboriginal communities. We conclude that the unusually high prevalence of HCV infection observed in some aboriginal communities in Taiwan could be ascribed to poor anti-septic medical practice derived from insufficiency of medical personnel and facilities in these communities as compared with the other regions in Taiwan at the time before twenty years ago.