The objective of this study was to describe the incidence and prevalence of hepatitis C infection among clients of a methadone program in Queensland. The clinical notes of clients receiving methadone for treatment of opiate dependence who first registered at the clinic after 1989 were perused for information about their serological status for hepatitis C and hepatitis B infections during a six-week period in 1994. We followed hepatitis C negative clients until August-September 1995. At study entry, 69 per cent of the clients were recorded as being hepatitis C positive. Of those who were negative, the seroconversion rate was 11 per 100 person-years. The high incidence and prevalence of hepatitis C among methadone clients emphasises the need for effective early intervention strategies to prevent the transmission of hepatitis C among injecting drug users.