Background.Hepatocellular carcinomas (HCC) that originate de novo in liver transplants without preceding HCC in the explanted organ have only rarely been reported. Because recent data demonstrated a mixed hepatocellular chimerism caused by the integration of host-derived stem cells, a study was conducted on the origin of tumor cells in de novo HCC.Methods.From two cases of de novo HCC arising in liver transplants after hepatitis B reinfection, tumor cells and non-neoplastic liver cells from the patient’s own liver and donor liver were isolated by laser microdissection, and highly polymorphic short tandem DNA repeats (STR) were investigated.Results.Isolated tumor cells revealed donor-specific STR genotypes that could clearly be discriminated from the genotype of the host.Conclusions.Hepatitis B virus-associated de novo HCC in liver transplants is of donor but not of host origin. The new technique described here can also discriminate between true recurrence of the original tumor and new recipient tumors.