We investigated the timing of the disappearance and reappearance of serum hepatitis C viral (HCV) RNA in patients with chronic hepatitis C during interferon treatment and follow-up. Serum samples were tested for HCV RNA by polymerase chain reaction in 62 patients with chronic hepatitis C treated with interferon-alpha for 24 weeks. We found that 17 patients obtained complete response, with absence of serum HCV RNA for 6 months after the treatment. Twenty-nine patients had a partial response, with reappearance of serum HCV RNA within 6 months of follow-up, and 16 patients were nonresponders who were positive for serum HCV RNA throughout the observation period. HCV RNA disappeared within 2 weeks of treatment in 31 patients, including all 17 (100%) complete responders and 14 (48.3%) of the 29 partial responders. The patients remaining positive for HCV RNA at the second week were 15 (51.7%) of the 29 partial responders and the 16 nonresponders. In all of the 29 partial responders, viremia recurred within 1 month after the treatment. These results indicate that the status of HCV RNA at the second week of treatment is a useful predictor of effective treatment, whereas status at the first month after cessation of treatment is useful for assessing the effectiveness of interferon itself.