It is known that respiratory function deteriorates with age. Endogenous damage to DNA is thought to contribute to the aging process. The mitochondrial oxidative phosphorylation system, a bio-engine, consists of five complexes, and 13 subunits of those complexes are biosynthesized from information encoded in mitochondrial DNA. Mitochondrial DNA is shown to have a much higher mutation rate than nuclear DNA. We examined the diaphragms obtained at autopsy from 34 humans, 23 men and 11 women, ranging in age from 25 to 85 yr, for mitochondrial DNA deletions using the polymerase chain reaction method. Multiple mitochondrial DNA deletions were detected particularly among the elderly; the number of deletions in those over age 70 was significantly higher than in those under age 40. The occurrence of a 3.4-kbp deletion of mitochondrial DNA increased with age, i.e., 0% of those under age 30, 20.0% of those in their forties, 25.0% of those in their fifties, 28.6% of those in their sixties, 72.7% of those in their seventies, and in all of those over age 80. The mutation was based on the directly repeated sequence, 5'-TCACCCC-3', which exists in both the CO3 gene and the ND5 gene. Replication impairment occurred at that directly repeated sequence, which caused the elimination of a genome between the CO3 gene and the ND5 gene, and information for biosynthesis of four subunits in complex I (ND3, ND4L, ND4, and ND5), one in complex IV (CO3), and five transfer RNA genes was missing.(ABSTRACT TRUNCATED AT 250 WORDS)