Recurrence of hepatocellular carcinoma after treatment is frequent. To study the clonal origin of the recurrent tumors, we examined five pairs of hepatocellular carcinomas resected from individual hepatitis B surface antigen carriers. Using integrated hepatitis B virus DNA as a marker, tumor clonality was determined by Southern blot analysis. In 2 cases the second tumor contained the same integrated viral DNA as the first one. In the other 3 cases, the clonality of the second cancer differed. We conclude that recurrent hepatocellular carcinomas originate from the first tumor in some cases but represent de novo neoplasms in others.