Hepatitis B virus (HBV) DNA integrates into human hepatocyte DNA. We have gathered the available data on the structure of the integrants from human hepatocellular carcinomas, and classified them into those that seem to represent primary integrants and those that are the products of secondary rearrangements. By means of structural analyses of the possible primary integrants, we deduced that the replication intermediates of the viral genome are the preferred substrates for integration. The integrated HBV DNA and the target cellular DNA are invariably associated with deletions, possibly reflecting the substrate for, and the mechanism of, the integration reaction. The target cell DNA sequence, as well as the target site of integration in chromosomes, seems to be selected randomly, suggesting that HBV DNA integration should bring about random mutagenic effects. Several samples recovered from hepatocellular carcinomas show that the integrated HBV DNA can mediate secondary rearrangements of chromosomes, such as translocations, inversions, deletions and (possibly) amplifications. Thus, HBV DNA integration causes multiple mutagenic effects. We argue that during hepatitis infection, the tendency of rearrangement of hepatocyte chromosomes is combined with the forcible turnover of cells. This is a constantly operating system for the selection of cells that grow better than average cells, possibly involving important steps in multistaged hepatocarcinogeneses.