We tested for antibodies to hepatitis B virus (HBV), hepatitis C virus (HCV), and human T lymphotropic virus type-I (HTLV-I) in 629 normal inhabitants of an adult T cell leukemia (ATL) endemic area and in patients with ATL, HTLV-I associated myelopathy (HAM), and hepatocellular carcinoma (HCC) from the same district. The prevalence of serological positivity for each virus was 28.0, 6.4, and 32.6%, respectively, among the 629 inhabitants. There was a positive association between the presence of anti-HCV and serological HTLV-I positive or negative status of these subjects (9.3% vs 5.0%). Conversely, there was no correlation between HBV and HTLV-I serologic prevalence. Only inhabitants positive for anti-HCV showed significantly high serum aminotransferase levels. The levels were not affected by superimposed HTLV-I infection among anti-HCV positives. Fifty three percent of HCC patients were positive for anti-HCV; 35% of whom were simultaneously positive for antibody to HTLV-I. On the other hand, only 2 ATL patients (4.2%) and 2 HAM patients (7.7%) had anti-HCV. These findings suggest that high serum aminotransferase levels are mainly caused by HCV infection and persons with HCV and HTLV-I double infections are at a high risk for the development of HCC but not ATL or HAM.