The southern district of N city (U area), Yamagata Prefecture, is highly endemic for hepatitic C virus (HCV) infection. Around 20% of the general population are positive for antibodies to HCV (anti-HCV). Community-acquired, acute non-A, non-B hepatitis was epidemic from 1967 to 1972 in this area. Our previous study revealed that these people are actually infected with HCV, but a relationship between this outbreak and the high positivity rate of anti-HCV in the U area has not been shown. We followed up 15 anti-HCV-positive individuals who developed hepatitis during the epidemic and used the serum collected to conduct molecular evolutional analysis to reveal the characteristics of the HCV epidemic in the U area. HCV genotypes in the U area were also analyzed. Phylogenetic analysis of the HCV core gene sequences showed that the subjects' HCV sequences were closely related and derived from the same cluster. All subjects were infected with HCV genotype 1b, which was frequently detected with a high positivity of over 80% of HCV-infected individuals in the U area. These results confirm that the community-acquired hepatitis C epidemic occurred around three decades ago through an unidentified route, and suggest that this episode may result in a continuing increase in the number of HCV-1b positive patients in this small area.