Background: Alanine aminotransferase (ALT) is frequently used as the sole biochemical marker for chronic hepatitis C (CHC), however, its value may be normal in cases with active disease. Recently, aspartate aminotransferase (AST) has been suggested as a useful predictor of liver pathology and conflicting results were obtained by using AST/ALT ratio to predict cirrhosis. Aims: To evaluate clinical utility of serum ALT and AST in CHC. Methods: The charts of 133 patients with CHC, whose ALT and AST were simultaneously tested from 1994 to 1996, were reviewed. ALT and AST were analyzed for both the entire cohort of patients and subgroups stratified for histopathology, age, gender, alcohol consumption, and risk factors of transmission. In 53 patients, the AST/ALT ratio was evaluated during interferon treatment. Results: The elevation of ALT significantly correlated with that of AST (r=0.79). The AST/ALT ratio increased with liver histological progression. The ratio >/=1 was predominantly in cirrhotic patients. During treatment the ratio increased. The AST remained elevated in eight of the 33 patients in whom the ALT had returned to normal during and after treatment. Conclusions: Both ALT and AST are useful markers for CHC. However, the AST may elevate alone, suggesting that measuring AST may be useful when the ALT is consistently normal. The AST/ALT ratio varies in different patients but increases with histological progression of fibrosis. An AST/ALT ratio >/=1 is highly suggestive of the presence of cirrhosis.