In order to investigate the effect of aging- and disease-associated deletion of mtDNA on cellular functions, we used cytoplasm fusion to construct a series of the cybrids harboring varying proportions of mtDNA with 4,977 bp deletion from skin fibroblasts of a patient with chronic progressive external ophthalmoplegia. The cybrids were grown in the Dulbecco's modified Eagle medium supplemented with 5% fetal bovine serum, 100 microg/ml pyruvate and 50 microg/ml uridine. The population doubling time was longer for the cybrids containing higher proportions of 4,977 bp-deleted mtDNA. In addition, we found that the respiratory function was decreased with the increase of mtDNA with 4,977 bp deletion in the cybrids. Since impairment of the respiratory system of mitochondria increases the electron leak of the respiratory chain, we further determined the oxidative stress in these cybrids. The results showed that the specific contents of 8-hydroxy 2'-deoxyguanosine and lipid peroxides of the cybrids harboring > 65% of the 4,977 bp-deleted mtDNA were significantly increased as compared with those of the cybrids containing undetectable mutant mtDNA. On the other hand, we found that the mitochondrial mass and the relative content of the mitochondrial genome in the cybrids harboring 4,977 bp-deleted mtDNA were higher than those of the cybrids containing only wild type mtDNA. The relative content of mtDNA was increased 17% and 30%, respectively, in the cybrids harboring 17% and 56% of mtDNA with 4,977 bp deletion. Moreover, both mitochondrial mass and mtDNA content were concurrently increased by treatment of the cybrids with 180 microM of hydrogen peroxide. Taken these findings together, we conclude that increase of mitochondrial mass and mtDNA are the molecular events associated with enhanced oxidative stress in human cells with impaired respiratory function caused by mtDNA deletion.