It is known that the 5%-10% of adults infected with hepatitis B virus (HBV) develop a chronic infection and that HBV infection acquired at birth by an hepatitis B surface antigen (HBsAg)/hepatitis B "e" antigen (HBeAg)-positive mother almost invariably leads to chronic infection. Little information is, however, available about the risk of HBV infection acquired in childhood becoming chronic. We have, therefore, studied the chronicity rate of HBV infection in the families of 60 consecutive HBsAg-positive chronic carrier children. Of parents 81.5% and 78.6% of children showed serological evidence of past or ongoing HBV infection. The chronicity rate was significantly higher among children (73.4%) than parents (35.6%). Such a high chronicity rate in these children was not correlated with vertical transmission, since this was reported in only 1.7% of them. It is noteworthy that the chronicity rate of HBV infection was not significantly different between children of HBsAg-positive mothers and those in whom infection must have been horizontally transmitted because their mothers were HBsAg-negative. Although the families studied represent a selected sample and the role of genetic factors could not be excluded, our results seem to show that the most important factor in determining the outcome of infection is the acquisition of hepatitis B during childhood.