To evaluate risk factors associated with intrafamiliar transmission of hepatitis C virus (HCV), 113 hepatitis C virus index subjects with chronic HCV infection and their 267 family contacts were studied from January 1994 to October 1995. Overall, 16 family contacts (6%) were positive for anti-HCV by ELISA II generation. The prevalence was 11.3% in spouses and 2.9% in other relatives (odds ratios: 4.2; 95% CI: 1.4-12.6). Spouses who had been married to the index cases longer than 20 years had a 7.5-fold risk (95% CI: 1.0-336.3) of HCV seropositivity as compared to those married less than 20 years. In univariate analysis HCV seropositivity was associated with surgical intervention, use of glass syringes and hospitalization. The results of multivariate logistic analysis showed that any parenteral exposure (odds ratios: 3.8; 95% CI: 1.2-12.8) and sexual contact with an anti-HCV index case (odds ratios: 3.0; 95% CI: 1.0-9.4) were both independent predictors of HCV seropositivity among household contacts of HCV positive index cases. These findings indicate that sexual contact and any parenteral exposure both play an independent role in the spread of HCV infection in the family setting.