The clinical progression of chronic hepatitis C is not uniform throughout the entire period of infection and is more rapid in patients with advanced histologic disease. Our study was designed to identify factors contributing to progression to cirrhosis and hepatocellular carcinoma by taking the entire period of infection into consideration. Two hundred thirteen patients with transfusion-associated hepatitis type C chronic liver disease were included in this study. They did not have either a history of antiviral therapy or any other potential causes of chronic liver disease except for transfusion. Hepatitis C virus genotype 1b was detected in 144 (68%) patients, followed by 2a in 51 (24%), 2b in 11 (5%), 1a in 4 (2%), and coinfection with 1b and 2a in 3 (1%). The log-rank test in the Kaplan-Meier method revealed that the cumulative percentage of cirrhosis-free or hepatocellular carcinoma-free patients became significantly lower as the transfusion age went up. Patient age at the time of transfusion was the only independent factor related to disease progression in multivariate analysis using Cox's proportional hazards model. Thus age at transfusion should be taken into consideration in designing the optimal follow-up schedule and therapy in patients with posttransfusion-associated chronic hepatitis C.