The study analyses data from clinical and genetic examination of 114 patients, as well as examination of cytological skin fibroblasts of 20 patients with hereditary optic neuropathy (HON). The clinical examination revealed HON symptoms in all study patients, primary damage of the retinal ganglion cells accompanied by swelling of the peripapillary retinal nerve fiber layer (RNFL) in the acute stage of the disease was observed in 47% of cases. MtDNA mutations that cause the development of Leber hereditary optic neuropathy (LHON) were detected in 73% of cases, including three frequent mutations in 59% of cases, rare and candidate mutations — in 14% of cases; nDNA mutations associated with autosomal dominant optic neuropathy (ADON) — in 6.1% of cases; mutations in the DNAJC30 nDNA gene that caused autosomal recessive optic neuropathy (ARON) — in 21% of cases. Among patients with a clinical picture of LHON, mtDNA mutations were found in 77.6% of cases, while mutations of the DNAJC30 gene of nDNA — in 22.4% of cases. Cytological studies using high-resolution respirometry confirmed the presence of mitochondrial dysfunction not only in the cells of patients harboring pathogenic mutations, but also of those harboring candidate mutations. An algorithm for clinical and genetic verification of HON together with a set of cytological studies allows identification of the mitochondrial genesis of the disease and is indispensable in confirming the pathogenicity of new or candidate mutations.