Recommended treatment for hepatitis C virus genotype 1 (HCV-1) patients is peginterferon plus ribavirin for 48 weeks. We assessed whether treatment duration of 24 weeks is as effective as standard treatment in HCV-1 patients with a rapid virological response (RVR; seronegative for hepatitis C virus [HCV] RNA at 4 weeks). Two hundred HCV-1 patients were randomized (1:1) to either 24 or 48 weeks of peginterferon-alpha-2a (180 μg/week) and ribavirin (1000–1200 mg/day) with a 24-week follow-up. The primary endpoint was a sustained virological response (SVR; seronegative for HCV RNA at 24-week follow-up). Overall, the 48-week arm had a significantly higher SVR rate (79%) than the 24-week arm (59%, P = 0.002). For 87 (43.5%) patients with an RVR, the 24-week arm had a lower SVR rate [88.9%; 95% confidence interval (CI): 80%–98%] than the 48-week arm (100%, P = 0.056). For 52 patients with low baseline viremia (<400,000 IU/mL) and an RVR, the 24-week arm had rates (CI) of relapse and SVR of 3.6% (−3%–11%) and 96.4% (89%–103%), respectively, which were comparable to those of the 48-week arm (0% and 100%) with difference (CI) of 3.6% (−7.2%–6.6%) and −3.6% (−14.3% to −0.6%), respectively. Multivariate analysis in all patients showed that RVR was the strongest independent factor associated with an SVR, followed by treatment duration, mean weight–based exposure of ribavirin, and baseline viral load. Conclusion: HCV-1 patients derive a significantly better SVR from 48 weeks versus 24 weeks of peginterferon/ribavirin even if they attain an RVR. Both 24 and 48 weeks of therapy can achieve high SVR rates (>96%) in HCV-1 patients with low viral loads and an RVR. (HEPATOLOGY 2008;47:1884–1893.)