Two pairs of young brothers from two families with histologically verified hepatocellular carcinoma (HCC) occurred with almost simultaneous onset in each family. In the first family, the boys were 5 and 7 years of age, respectively. Both of them, their parents, and a younger sister were hepatitis-B-surface-antigen (HBsAg)-positive. In the second family, the two brothers were 9 and 7 years of age, respectively. Both patients, their mother, and maternal grandmother were HBsAg-positive. The fraternal occurrence of HCC in children under the age of 10 years has not been reported in the literature. The pattern of HBsAg on these families suggests the HCC cases were infected by their mothers, most possibly in the perinatal period, and is additional support for the important role of hepatitis-B virus in the etiology of HCC. The very early and almost simultaneous onset, however, suggests that additional genetic and/or environmental factors are also important.