Alpha interferon therapy of chronic hepatitis C is typically accompanied by a biphasic decrease in hepatitis C virus (HCV) RNA levels: an initial rapid decline during the first 24 to 48 hours, and a second more gradual decline during the following weeks. The rate of second-phase decline correlates with ultimate response to interferon treatment. Thus, assessment of early virological response (EVR) may predict outcome. Data from 2 large clinical trials of peginterferon and ribavirin were combined and analyzed to determine the optimal definition of an EVR which, if not achieved, was associated with a low likelihood of a sustained virological response (SVR). A fall in HCV RNA level to undetectable or by at least 2 log(10) units after 12 weeks was found to be the optimal definition of an EVR. Among 965 patients, 778 (80%) achieved an EVR by week 12, including all except 1 patient with genotypes 2 or 3. Among 187 patients without an EVR, only 3 (1.6%) had an SVR. These findings suggest that patients with genotype 1 who do not achieve an EVR should stop treatment after 12 weeks. Use of an early stopping rule reduces treatment costs by at least 16% and avoids the inconvenience and side effects of treatment in the 19% of patients without an EVR who have little chance of a lasting virological response.