The northern district of R Town, Yamagata Prefecture, Japan, experienced an epidemic of acute hepatitis C during the 6 years following 1967. A mass survey health examination for hepatitis C virus (HCV) infection was performed on the inhabitants of this district aged 6 or over (4, 655 people). Of the 3, 094 inhabitants (66.5%) examined, 602 (19.5%) were anti-HCV antibody (anti-HCV) positive. Of the 602 anti-HCV positive subjects, 444 (73.7%) showed an anti-HCV cutoff index of over 4.0 (high titer), 54 (9.0%) showed 3.0-4.0 (middle titer), and 104 (17.3%) showed an index of under 3 (low titer). One hundred and sixty subjects, randomly selected, were examined for HCV RNA by nested polymerase chain reaction, and positive reactions were detected in 125 subjects (78.1 %). The anti-HCV positive rate differed greatly between subjects younger than 40 years (18/1, 289 ; 1.4%) and those 40 or over (584/1, 805 ; 32.4%). Those who had lived in this district since before the acute hepatitis C epidemic had a high rate of positive anti-HCV. Among the 47 hamlets in this farming district, the hamlet with the highest rate (71.4%) was surrounded by other hamlets where the rates decreased as the distance from the high rate hamlet increased. The study suggested a marked regional accumulation of HCV infection in this district and an age-related difference in HCV infection rates. The cause of the accumulation was not identified, although the HCV infection in this district may have been spread during or before the acute hepatitis C epidemic and produced many HCV carriers. J Epidemiol, 1996 ; 6 : 1-7.