Psychiatric disorders or drug addiction are often regarded as contraindications against the use of interferon alfa (IFN-alpha) in patients with chronic hepatitis C. Our aim was to obtain prospective data on adherence to as well as efficacy and mental side effects of treatment with IFN-alpha in different psychiatric risk groups compared with controls. In a prospective trial, 81 patients with chronic hepatitis C (positive hepatitis C virus[HCV] RNA and elevated alanine aminotransferase[ALT] level) and psychiatric disorders (n = 16), methadone substitution (n = 21), former drug addiction (n = 21), or controls without a psychiatric history or drug addiction (n = 23) were treated with a combination of IFN-alpha-2a 3 MU 3 times weekly and ribavirin (1,000-1,200 mg/d). Sustained virologic response (overall, 37%) did not differ significantly between subgroups. No significant differences between groups were detected with respect to IFN-alpha-related development of depressions during treatment. However, in the psychiatric group, significantly more patients received antidepressants before and during treatment with IFN-alpha (P <.001). Most of those who dropped out of the study were patients with former drug addiction (43%; P =.04) compared with 14% in the methadone group, 13% in the control group, and 18% in the psychiatric group. No patient in the psychiatric group had to discontinue treatment because of psychiatric deterioration. In conclusion, our data do not confirm the supposed increased risk for IFN-alpha-induced mental side effects and dropouts in psychiatric patients if interdisciplinary care and antidepressant treatment are available. Preexisting psychiatric disorders or present methadone substitution should no longer be regarded as contraindications to treatment of chronic hepatitis C with IFN-alpha and ribavirin in an interdisciplinary setting.