To evaluate the intrafamilial transmission of hepatitis C virus (HCV) 104 index patients with type C chronic liver disease and their 307 family contacts were interviewed. After a questionnaire on the risk factors of parenteral exposure, blood samples were obtained and tested for liver biochemistry and anti-HCV antibody by enzyme-linked immunosorbent assay (Abbott II). Overall, 52 family contacts (17%) were positive for anti-HCV, indicating a higher anti-HCV prevalence among family contacts than among the general population in Taiwan. The anti-HCV prevalences in parents, spouses, children, and other contacts of the patients were 54% (14/26), 28% (25/91), 6.9% (10/143), and 6.4% (3/47), respectively. The contacts of index patients had increasingly greater risk of HCV infection when they became older and had lived longer with index patients. All family contacts were divided into two groups categorized by whether the index patients had or did not have a history of parenteral exposure. Among 126 family contacts of the 42 patients without parenteral exposure, blood transfusion and surgery were the factors significantly associated with HCV infection in these family contacts (odds ratio = 7.26, 95% confidence interval = 2.32–32.67; odds ratio = 3.95, 95% CI = 1.29–12.11, respectively). Risk factors were not significantly associated with HCV infection among 181 family contacts of the 62 index patients with parenteral exposure. It is concluded that the index patients without parenteral exposure appeared to have acquired the disease from HCV-infected family members with risk factors. Most of the index patients had a history of parenteral exposure and in turn served as the source of the disease for family members. © 1994 Wiiey-Liss, Inc.