Using PCR technique and restriction mapping, we analyzed liver mitochondrial DNA (mtDNA) of 2 stillborn babies and 55 Chinese subjects from 27 to 86 years old and blood cell mtDNA from 20 subjects of various ages. An ageing-associated 4,977-bp deletion was detected between nucleotide position 8,469 and 13,447 (or between 8,482 and 13,460) in the liver mtDNA of older subjects. In the region containing the junction fragment, we observed a 13 bp repeat "ACCTCCCTCACCA". Moreover, the incidence of the deleted mtDNA of each of the study subjects was found to increase with age. The deletion was found in 5 out of 8 patients of the 31-40 age group and 9 out of 11 patients of the 41-50 age group, and in all the patients over 50 years old. The deletion was not observed in either the mtDNA of the liver of the stillbirth or the blood cells of subjects of all the age groups. These results support our previous contention that liver mitochondrial respiratory functions decline with age and the hypothesis that continuous accumulation of mitochondrial DNA mutation is an important contributor to ageing process.