The intrafamilial transmission pattern of hepatitis C virus (HCV) was examined in 118 family members of 61 index patients with type C chronic liver disease using anti-HCV antibodies and HCV RNA assay. The study subjects consisted of eight parents, 49 spouses, 50 children, eight siblings and three other relatives. The positivity rates of anti-C100, anti-JCC, second-generation anti-HCV and HCV RNA were 6.8, 12.7, 12.7 and 11.0%, respectively. Positivity in one or more anti-HCV antibody assay was detected in 3/24 (12.5%) father-child pairs, 3/17 (17.6%) mother-child pairs, 2/8 (25%) sibling pairs, 6/38 (15.8%) husband-wife pairs and 2/13 (15.4%) wife-husband pairs. In spouses, positivity for anti-HCV antibody or HCV RNA was observed after 40 years of age. None of 11 spouses married < 15 years was positive for any anti-HCV assay or HCV RNA. In spouses whose age was > 50 years and duration of marriage was > 25 years, anti-HCV or HCV RNA was frequently detected (32.0%). However, when seven pairs involving four spouses, one mother-daughter pair and two sibling pairs were subtyped, the same HCV subtypes were found in only four pairs (type II in three pairs and type III in one pair). Further, the agreement rate between anti-HCV and HCV RNA was > 90%. These results suggest that intrafamilial transmission of HCV, revealed by the subtyping method, is considered lower than the percentage of positivity for anti-HCV antibodies or HCV RNA in family members of patients with type C chronic liver disease.(ABSTRACT TRUNCATED AT 250 WORDS)