BACKGROUND: A study of the intrafamilial transmission of the hepatitis B virus (HBV) and its relationship with the viral replication and epidemiological factors. METHODS: The intrafamilial transmission of 106 chronic carriers of HBV was evaluated: 79 were asymptomatic carriers (AC) and 27 had chronic liver disease (CLD). Overall 347 relatives of the first group individuals and 112 of the second group were investigated. In the index cases, all HBV markers were investigated, and also DNA-HBV in those with CLD. In the relatives, HBsAg, HBsAc and HBcAc were investigated. Also, a survey to evaluate the influence of socioeconomic and cultural factors was also carried out. RESULTS: The prevalence of markers was significantly higher in the relatives of patients with CLD (HBcAg, HBcAc and evidence of contact) followed by AC and controls. The most infective relation in AC was that of other contacts with significant differences from the mother-child relationship (HBsAc p less than 0.003, HBcAc p less than 0.01, and evidence of contact p less than 0.001). By contrast, in CLD the most infective relation was mother-child. The mother-child relation was more infective than the father-child one (HBsAg p less than 0.05, HBcAc p less than 0.03, and evidence of contact p less than 0.02). Regarding viral replication, it was found that the HBeAg and DNA positive patients were more infective. The prevalence of HBcAc and the evidence of contact increased with the time of living together of spouses. Finally, it can be stated in a general sense that, according to the results of the survey, the socioeconomic factors have a small influence on the familial transmission. CONCLUSIONS: The relatives of HBV carriers, particularly in the case of HC with high replication, have a high risk of transmission. Thus, their investigation and subsequent vaccination is mandatory.