To study the intrafamilial transmission of hepatitis C virus (HCV), 36 family members of 16 patients with anti-HCV (anti-C100-3)-positive chronic liver disease were screened for anti-HCV by an enzyme-linked immunosorbent assay (ELISA). Clusters of anti-HCV-positive individuals were observed in 2 of 16 families (12.5%). Four of 35 family members (11.4%) with no history of blood transfusion were positive for anti-HCV. Two of 17 offspring (11.8%) of anti-HCV-positive females were positive for anti-HCV, while 1 of 5 spouses (20.0%) was positive for anti-HCV. These data suggest that intrafamilial transmission is one of the possible routes of infection for HCV.