Hepatitis C is the leading cause of chronic liver disease in industrialized countries, and is the most common indication for liver transplantation. In the Western world, the absolute majority of cases of Hepatitis C Virus (HCV) infection are related to the use of injectable narcotic drugs. Most injecting drug users contract HCV infection within the first years after starting injecting drug use. The aim of this study is to study hepatitis C in a cohort of patients registered in clinics providing maintenance therapy for opiate dependence in three metropolitan areas of Sweden. The cohort is defined as all patients registered in these three clinics at the date of study initiation. The study contains four parts: