Quantitative and qualitative changes in circulating extracellular DNA (ec-DNA) of blood plasma are considered as markers for diagnosis and prognostic of tumor pathology. We investigated the content of mutant copies of the circulating extracellular mitochondrial DNA (ec-mtDNA) in blood plasma (using the enzymatic method, based on the cleavage of DNA with unpaired bases by CEL-I endonuclease) in 8 patients with lung cancer before and after radiotherapy, as well as in healthy young and elderly donors. It was found that in the plasma of healthy elderly donors share of ec-mtDNA with mutations (consisting of total circulating DNA) is much greater, than that of young donors. On the other hand, in the plasma of lung cancer patients (aged 70-76 years) before radiotherapy a substantial increase in the share of ec-mtDNA with mutations, compared with that of healthy elderly donors. Following radiotherapy, patients with lung cancer found a twofold increase of the proportion of ec-mtDNA with mutations in the total circulating plasma DNA. This increase is largely, perhaps due to the release of ec-mtDNA with mutations from dying tumor cells and cells damaged by normal tissues.