We conducted an epidemiological study to investigate the routes of transmission of hepatitis C virus (HCV) infection in an area endemic for HCV. Subjects were the 857 adult inhabitants of K area, Japan. The prevalence of antibody to HCV (anti-HCV) was 26.3%. The HCV genotype 1b was the most prevalent (89.3%) among the anti-HCV positive subjects. Molecular evolutionary analysis, based on the nucleotide sequences of the HCV core region from 23 participants with type 1b, showed that the isolates were distributed into more than 1 group. Multivariate regression analysis demonstrated that an age over 40 years, a history of blood transfusion, the presence of antibody to hepatitis B core antigen (anti-HBc), or a history of surgery were each independently associated with the presence of anti-HCV. No significant differences in the presence of anti-HCV prevalence were observed between the wives of men positive for anti-HCV (33.3%) and age-matched women (36.4%), or between the husbands of women positive for anti-HCV (36.4%) and age-matched men (38.3%), from the same area. Findings suggest that the various strains of HCV type 1b were transmitted via medical procedures, not by the intrafamilial route.