The Major Depression Inventory (MDI) was used to estimate the value of routine medical interviews in diagnosing major depression among patients receiving peginterferon alfa-2a and ribavirin therapy for chronic hepatitis C virus (HCV) infection (n = 325). According to criteria from the MDI and Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), 19 patients (6%) had major depression at baseline. An additional 114 (37%) developed depression while on HCV combination therapy, with baseline MDI score and female sex independently predicting the emergence of major depression during treatment in a multivariate analysis. Only 36 (32%) of the 114 patients developing major depression according to MDI/DSM-IV criteria were correctly diagnosed during routine medical interviews. The emergence of major depression frequently led to premature discontinuation of peginterferon/ribavirin therapy, and an on-treatment MDI score increment exceeding 30 points (i.e., a validated marker of idiopathic DSM-IV major depression) was correlated with impaired outcome of HCV therapy (P = 0.02). This difference was even more pronounced among patients with an on-treatment increase in MDI score greater than 35 points (P = 0.003). Conclusion: We conclude that (1) depressive symptoms among patients undergoing HCV therapy are commonly overlooked by routine clinical interviews, (2) the emergence of depression compromises the outcome of HCV therapy, and (3) the MDI scale may be useful in identifying patients at risk for treatment-induced depression.