A prospective follow-up study on hepatitis C virus (HCV) infection was conducted in seven haemodialysis units from April 1990 to March 1995. A total of 634 patients were undergoing maintenance haemodialysis in the seven units. Of those, 302 patients participated in the follow-up study; 179 were initially HCV antibody negative and 123 were initially positive. Nine of the 179 initially negative patients became positive for HCV antibody during the follow-up period. In accordance with the appearance of HCV antibody, indicating new infection of HCV, all nine of these patients were diagnosed with HCV viraemia. As no other routes were apparent, HCV infection in all nine patients was likely due to nosocomial transmission. Prevalence of HCV antibody at the start of follow up was significantly higher (P < 0.001) in haemodialysis units A-C (37.9%) than in haemodialysis units D-G (17.0%). Incidence of new HCV infection was significantly higher (P = 0.005) in the former units (2.2% per year) than in the latter (0.2% per year). Ten of the 123 patients who were initially positive for the HCV antibody exhibited a loss of reactivity during the follow-up period; of these 10 patients, nine were negative for HCV-RNA from the start of the study. In conclusion, the incidence of new HCV infection seen in patients undergoing haemodialysis suggests that their risk of acquiring HCV infection is directly related to the prevalence of HCV antibody positive patients being treated in the units.