We have studied the prevalence, seroconversion rate of anti-hepatitis C virus (HCV), and the transmission of HCV in a cohort of individuals living in a hyperendemic area of HCV in Japan. We investigated 509 subjects, of which 375 could be studied again after 5 years. A remarkable high prevalence of anti-HCV (23.4-24.0%) was observed. Of 287 subjects negative at the first examination in 1990, 4 became positive until the second in 1995 (seroconversion rate: 0.28% per year). Furthermore, we investigated the route of transmission in HCV seroconverted subjects through a detailed interview. All of the HCV seroconverted subjects had past histories of medical treatment. Seroconversion rates of HCV in a hyperendemic area of HCV, were extremely high. Medical treatment was considered to be a causative route of HCV transmission.