We have investigated the relationship between serum alanine aminotransferase (ALT) and hepatitis C virus (HCV) RNA in the assessment of responses to interferon (IFN) therapy in chronic HCV infection. Data from 704 patients with HCV infection who were randomized to receive consensus IFN-alpha (CIFN) 3 micrograms (n = 232 patients) or 9 micrograms (n = 232 patients), or IFN-alpha 2b 3 million units (MU) (n = 240 patients), were used for these analyses. All patients were treated three times weekly. Hepatitis C viral RNA (HCV RNA) was determined by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) with a lower limit of detection of 100 copies ml-1. Of patients with normal serum ALT concentrations, 53% (120/225) had undetectable HCV RNA at the end-of-treatment period and 47% (51/109) had undetectable HCV RNA at the end of the post-treatment observation period. In contrast, of the patients with undetectable HCV RNA, 75% (120/161) and 84% (51/61) had normal serum ALT activities at the end-of-treatment and post-treatment observations periods, respectively. The majority of patients with undetectable HCV RNA had normal ALT values. In contrast, only half of the patients with normal ALT values were negative for HCV. End-of-treatment HCV RNA response also better predicted sustained virological response than did end-of-treatment ALT response.