We studied the prevalence of antibodies against hepatitis C virus (anti-HCV) among 530 household contacts of 225 anti-HCV-positive subjects (index cases). Twenty-six (4.9%) relatives had anti-HCV, a proportion higher than that found among blood donors (175 of 22,435; 0.78%) (p less than 0.001). We did not find any differences regarding the type of relation with the index case (sexual or nonsexual). The prevalence of anti-HCV increased with the age of the relatives, with the contact time with the index case, and with the time of exposure to HCV. On the other hand, the anti-HCV was associated mainly with the existence of cirrhosis or hepatocellular carcinoma in the patient. We concluded that intrafamilial transmission may be an important mechanism in the spread of HCV.