The progression from chronic hepatitis C virus (HCV) infection to hepatocellular carcinoma (HCC) has been reported. We evaluated whether co-infection with the human T-lymphotropic virus type I (HTLV-I) might be associated with this transition in a cross-sectional analysis of 127 patients with HCV-chronic hepatitis (mean age = 51.7) and 43 patients with HCV-associated HCC (mean age = 62.4); the seroprevalence of anti-HTLV-I was 9.5% and 30.2%, respectively. For subjects 50 years or older, the seroprevalence of anti-HTLV-I in HCC patients was 13/41 (31.7%) which was significantly higher than that in chronic hepatitis patients (6/82, 7.3%) (P = 0.001). The relative risk (RR) of association was 12.8 (P = 0.0004) among the males, however, no association was evident among the females, RR = 1.3 (P = 0.80). The increased prevalence of HTLV-I positivity among the HCC cases could not be attributed to a higher rate of prior transfusion. These data suggest that co-infection with HTLV-I may contribute to the development of HCC among patients with HCV-induced chronic liver diseases in a highly HTLV-I-endemic area.