Fifty-one children, aged between 3 and 16 years, were diagnosed to have hepatocellular carcinoma (HCC) in the last 15 years. Serum hepatitis B surface antigen (HBsAg) was positive in all the latter 34 HCC children checked by radioimmunoassay, and was positive in five of the 17 earlier HCC children studied by double immunodiffusion method. Serum HBsAg was studied in 31 mothers of the latter 33 HCC children, including two pairs of affected siblings and was positive in 29 (94%). The positive rate was much higher than that (50%) of the mothers of control HBsAg carrier children (P less than 0.001). The serum HBsAg positive rate was also higher in the siblings of HCC children than that of the control group. On the contrary, the serum HBsAg positive rate was not different between the fathers of HCC children and that of the control group. Our observation demonstrated that transmission of hepatitis B virus from the mothers during the perinatal period or early childhood is the most important mode of hepatitis B virus infection in HCC children in Taiwan.