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To determine the relationship between methadone maintenance therapy (MMT) and hepatitis C (HCV) seroconversion among illicit drug users.A generalized estimating equation model assuming a binomial distribution and a logit-link function was used to examine for a possible protective effect of MMT use on HCV incidence.Data from three prospective cohort studies of illicit drug users in Vancouver, Canada between 1996 and 2012.A total of 1004 HCV antibody-negative illicit drug users stratified by exposure to MMT.Baseline and semi-annual HCV antibody testing and standardized interviewer-administered questionnaire soliciting self-reported data relating to drug use patterns, risk behaviors, detailed socio-demographic data and status of active participation in an MMT program.One hundred and eighty-four HCV seroconversions were observed for an HCV incidence density of 6.32 [95% confidence interval (CI) = 5.44-7.31] per 100 person-years. After adjusting for potential confounders, MMT exposure was protective against HCV seroconversion [adjusted odds ratio (AOR) = 0.47; 95% CI = 0.29-0.76]. In subanalyses, a dose-response protective effect of increasing MMT exposure on HCV incidence (AOR = 0.87; 95% CI = 0.78-0.97) per increasing 6-month period exposed to MMT was observed.Participation in methadone maintenance treatment appears to be highly protective against hepatitis C incidence among illicit drug users. There appears to be a dose-response protective effect of increasing methadone exposure on hepatitis C incidence.