Previous trials investigating the efficacy of treatment durations shorter than the standard of 24 weeks for chronic hepatitis C virus (HCV) genotype 2/3 infections have yielded discordant results. The aims of this investigator-initiated phase III study were to compare the efficacy of 12 or 24 weeks of treatment and to identify patients suitable for short-term therapy. Three hundred eighty-two genotype 2/3–infected patients [intention-to-treat (ITT) population] at 31 centers in Denmark, Finland, Norway, and Sweden were randomized to 12 or 24 weeks of peginterferon α-2a (180 μg/week) plus ribavirin (800 mg/day). Twelve weeks of therapy was inferior to 24 weeks in the ITT population (sustained viral response [SVR] rates: 59% versus 78%, P < 0.0001) and in the subgroups of patients infected with genotype 2 (56% versus 82%, P = 0.006) or 3 (58% versus 78%, P = 0.0015). These differences were observed regardless of the fibrosis stage. Age and HCV-RNA levels on days 7 and 29 were independent predictors of SVR. Short-term treatment was useful in patients < 40 years old, especially if HCV-RNA was undetectable on day 29, and also in patients ≥ 40 years old, provided that HCV-RNA was below 1000 IU/mL on day 7 in addition to being undetectable on day 29. If neither of these two criteria were met for patients ≥ 40 years old, 24 weeks of therapy was superior (P < 0.0001). Conclusion: Peginterferon/ribavirin treatment for 12 weeks in HCV genotype 2/3 infection is overall inferior to 24 weeks of treatment but may be useful in some patients with a rapid initial clearance of virus. (HEPATOLOGY 2008.)