Interferon (IFN)-alpha plus ribavirin is an effective treatment for hepatitis C virus (HCV) infection, but is associated with a high rate of depression. Depression has been linked to a worse outcome in multiple medical disorders including viral illnesses. We examined whether increased symptoms of depression during IFN-alpha/ribavirin therapy were associated with a reduced treatment response as assessed by clearance of HCV. Depressive symptoms were evaluated in 102 HCV-infected patients at baseline and after 4, 8, 12, and 24 weeks of pegylated IFN-alpha-2b plus ribavirin therapy using the Zung self-rating depression scale (SDS). Viral clearance was determined at 24 weeks by polymerase chain reaction (PCR). Only 34% of subjects (10 out of 29) with a 20-point or greater increase in SDS Index score were HCV PCR negative at 24 weeks, compared to 59% (24 out of 41) of patients with a 10-19 point increase in SDS Index and 69% (22 out of 32) of patients with a less than 10 point increase (chi2=7.6, df=2, p <0.05). In addition, a 20-point or greater increase in SDS Index score during IFN-alpha/ribavirin therapy significantly predicted failure to clear virus when considered alone [crude odds ratio (OR), 3.2; 95% confidence interval (CI), 1.3-8.0; p <0.01] or when controlling for other factors that affected IFN-alpha treatment response (adjusted OR, 3.6; 95% CI, 1.3-9.5; p=0.01). These preliminary findings suggest that individuals who experience significant increases in depressive symptoms during IFN-alpha/ribavirin therapy may be less likely to clear virus, highlighting the importance of identifying and treating depressive symptoms in this patient population.