Hepatitis C is the most commonly notified disease in Australia. In 1998 the Hepatitis C Virus Projections Working Group (HCPWG) estimated that there were approximately 210,000 people who had been infected by hepatitis C virus (HCV) in Australia by 2001. Population-based serosurveys are required to validate this estimate. Here we estimate HCV prevalence on the basis of HCV antibody seroprevalence in the Australian national serosurvey. Between 1996 and 1998, 2,800 sera opportunistically collected from pathology laboratories throughout Australia were tested for HCV antibody. National HCV notifications reported from 1991 through 1998 were also assessed. Eighty-one sera were HCV antibody positive, giving an age standardised prevalence of 2.3 per cent (95% CI 1.8%-2.9%). The 20-24 year age group had the highest HCV prevalence, 5.3 per cent (95% CI 3.3%-8.1%) and the male to female ratio was 1.8:1.0. Approximately 111,000 HCV notifications were received from 1991 through 1998. HCV prevalence estimated by the serosurvey is approximately three times higher than cumulative HCV notifications. Age and sex distributions of seroprevalence are broadly consistent with cumulative notification data. These distributions are consistent with the majority of HCV infections in Australia being transmitted by injecting drug use. Very low age specific seroprevalence estimates in the over 50 years age group indicate that there is not a large pool of undiagnosed infection in this age group. The serosurvey provides an estimate of Australian HCV prevalence and baseline data to determine incidence trends, both of which are required for health-care planning.