Recently, mitochondrial DNA (mtDNA) mutations has been clarified to be linked with the genesis of various diseases. We found that genetic impairment exists in patients with cardiomyopathy, and that responsible mtDNA locus was estimated. MtDNA were amplified from autopsied cardiac specimens from two patients with cardiomyopathy, using the polymerase chain reaction (PCR). The existence of multiple populations of differently deleted mtDNAs was demonstrated. Direct sequencing of major mutant mtDNA revealed that a 12-bp directly-repeated sequence of 5'-CATCAACAACCG-3' was involved in the 7,436-bp deletion between the ATPase 6 gene and the D-loop region. We also found this deletion in aged controls, but the amount of the deletion in these patients was less than that of age-matched controls. According to out findings, it is concluded that defects in molecular assembly of the energy transducing system in mitochondria, derived from mtDNA mutations, might be an important contributor to cardiomyopathy.