The effectiveness of extending treatment duration as response guided therapy was previously reported for chronic hepatitis C (CHC) genotype 1, but is still controversial for genotype 2. The present study is a retrospective cohort study to investigate the effectiveness of extending treatment duration in therapy with pegylated interferon and ribavirin for patients with CHC genotype 2 by focusing on the timing at which patients obtained undetectable HCV RNA. A total of 306 patients who obtained undetectable HCV RNA by week 24 of treatment and completed 24 weeks of treatment were enrolled. Rapid virological response (RVR) to standard therapy was achieved by 122 patients (51%), and 89% of them obtained sustained virological response (SVR), while 69% of non-RVR patients achieved SVR. Non-RVR patients with undetectable HCV RNA at week 8, and insufficient adherence < 80% pegylated interferon and ribavirin during the first 24 weeks, significantly improved their SVR rate by extended therapy. Among patients receiving extended therapy, drug adherences did not differ between SVR and non-SVR patients, indicating that extending treatment duration might compensate for insufficient antiviral effects due to insufficient drug adherences. This finding might be useful in creating a guideline for extending treatment duration for patients with CHC genotype 2.