A study was undertaken to establish the risk of family contacts of HBsAg carriers acquiring a hepatitis B virus (HBV) infection. About one-third of all household contacts of asymptomatic HBsAg carriers had signs of past or ongoing HBV infection. Family contacts of HBsAg carriers with high numbers of circulating Dane particles were shown to have a higher risk of developing HBV infection than family contacts of HBsAg carriers without serological evidence of HBV synthesis. The probability of acquiring HBV infection was not different between spouses, parents, children, and brothers and sisters, respectively of asymptomatic HBsAg carriers.