The aim was to demonstrate adherence to treatment has been suggested to enhance rates of sustained response in patients with hepatitis C. In this study, we evaluated the effect of drug dosage reduction or the duration of the expected therapy in patients treated with interferon (IFN)-alpha2b plus ribavirin. Virologic response rates were re-analysed according to compliance to therapy in (i) 301 naive and (ii) 142 nonresponders to previous IFN therapy treated with either IFN 5 MU TIW for 8 weeks followed by IFN 3 MU TIW for 40 weeks plus ribavirin or IFN 3 MU QD for 16 weeks followed by IFN 3 MU TIW for 24 weeks plus ribavirin. Patients were separated into those who adhered to > or =80% of their intended treatment schedule (dose of both drugs and duration) and those who did not. Compliance to treatment resulted in significantly higher response rates in both groups of patients: 43.93% compared with 6.90% of noncompliant naive patients and 30.77% compared with 10.53% of nonresponder patients. Compliance to treatment was found to have a similar effect when the results were analysed according to HCV genotype. Our findings suggest that compliance to treatment for > or =80% of the intended treatment schedule results in significantly higher sustained response rates in both naive and nonresponder patients. Consequently, every effort should be made to improve patient adherence to therapy.