The sample size calculation is based on the primary outcome variable of HCV incidence among OST patients. The literature reports an HCV incidence of about 4/100 person-years +/- 2 new infections per 100 person-years in this target group. For a partial sample size of (at least) 32 patients with HCV seroconversion and an observation period of 1 year, 800 HCV antibody-negative patients are needed as an initial group for observation. Assuming an HCV antibody prevalence of 68% among OST patients, 2500 patients are needed in total (32% corresponds to N = 800).