Hepatitis B virus integration in hepatitis B virus-related hepatocellular carcinoma in childhood. In Taiwan, hepatocellular carcinoma is one of the major malignancies in children between 5 and 14 yr of age. We studied the status of hepatitis B virus DNA in the hepatocellular carcinoma and nontumorous liver tissues of eight children with positive serum HBsAg and maternal HBsAg. The hepatocellular carcinoma tissues from five of the eight children showed integration of hepatitis B virus DNA into host cellular DNA sequences. A pattern of single-site integration in four children and a multiple-site integration pattern in one child were demonstrated. In the remaining three children, hepatitis B virus DNA could not be demonstrated in the tumor tissues. Using subgenomic fragments of the hepatitis B virus genome as probes, we found that the X gene fragment and the surface antigen gene fragment were the most conserved sequences. The single-site integration of hepatitis B virus DNA in childhood hepatocellular carcinoma may have hit the critical region, resulting in insertional mutagenesis and early development of hepatocellular carcinoma. With a short incubation period and less exposure to environmental carcinogens during early life, childhood hepatocellular carcinoma may provide a good model to study the carcinogenic potential of hepatitis B virus. 