To understand the role of mitochondria in carcinogenesis, we compared the amount of deleted mtDNAs between human hepatic tumors and surrounding cirrhotic portion of the liver of ten patients by using polymerase chain reaction (PCR). Multiple mtDNA deletions were detected in cirrhotic portion, but no deletions were detected in the tumor portion. Direct sequencing of the fragments revealed a 7,079-bp deletion (nucleotide position 8,992-16,072) involving no direct repeated sequences and a 7,436-bp deletion (position 8,649-16,084) involving a 12-bp directly repeated sequence of 5'-CATCAACAACCG-3' exists in both the ATP6 gene and the D-loop region. These mtDNA mutations could be one of the endogenous factors that induce somatic mutations in nuclear genome and etiologically contribute to human carcinogenesis.