Amplification of one of the frequently deleted regions of myocardial and lymphocytic mitochondrial DNA (mt DNA) from patients with dilated cardiomyopathy (DCM), acute myocardial infarction (AMI) and normal controls was carried out with polymerase chain reaction (PCR). The results showed that multiple deletions exist in mtDNA of both patients and normal controls. 7.8 kb deletion was detected in myocardial mtDNA of 2 patients with AMI and 10 patients with DCM; 7.47, 8.10, 8.32 and 8.48kb deletions were found in lymphocytic mtDNA of 3 patients with AMI and 25 patients with DCM as well as 14 normal controls. The incidence and abundance of the 7.47kb mtDNA deletion increased with age of all the subjects studied. Quantitative analysis showed that mutated mtDNA accumulated up to 10% of total mtDNA-only in some older patients. Single strand conformational polymorphism analysis of the amplified 8,530-9,333 fragment allowed us to find a point mutation in a patient with DCM. Clinical significance of the mutations in mtDNA was discussed.