Although injection drug users represent the majority of new and existing cases of infection with hepatitis C virus (HCV), many lack access to treatment because of concerns about adherence, effectiveness, and reinfection. On the basis of a small but growing body of evidence showing that injection drug users can undergo treatment for HCV infection successfully, the 2002 NIH Consensus Statement on Hepatitis C has recommended that substance users be treated for HCV infection on a case-by-case basis. In this study, all patients will receive pegylated interferon alfa-2b (PegIFN-2b) and ribavirin according to European labeling; one cohort of participants will also be receiving substitution therapy (opioid medicines with long-lasting effects [methadone + buprenorphine] or morphine).