A study was made of the routes of transmission of HCV in 106 carrier patients and the prevalence of antibodies to HCV antigen (HCV Ab) in 132 family contacts (mothers, sexual partners and children of carrier women) attended at a Primary Care Center in Madrid. The following data were obtained through a personal interview: age, gender and possible routes of contagion. Also, ALT and serologic status to HBV. Household contacts were interviewed and HCV Ab status obtained. Only in 21 patients (19.8%) was no risk factor found ("sporadic cases"). Eighty-one individuals (76.4%) and 13 (12.3%) had percutaneous transmission and sexual transmission risk factors, respectively. Out of 18 studied mothers, two (11.1%) had HCV Ab. The prevalence of HCV Ab in household contacts was 8.35%. These results suggest that the proportion of sporadic cases decreases significantly when a thorough investigation is carried out in carrier patients. The percutaneous route is responsible for most of HCV infections. The prevalence of HCV Ab among household contacts appears to be higher than among general population.