The article is devoted to the clinical polymorphism of mitochondrial diseases by the example of A3243G mutation in mitochondrial DNA. The article also discusses clinical criteria and an algorithm for the diagnosis of mitochondrial diseases.Material and methods. Тhree families with A3243G mutation in mitochondrial DNA are presented. All patients underwent clinical neurological examination, instrumental examination (ECG, Echo-CG, MRI and CT of the brain, EEG, needle and stimulation electromyography, audiometry), biochemical study of the level of lactic and pyruvic acids in the blood before and after exercise, muscle biopsy.Results. Аll patients had myopathy, exercise intolerance, sensorineural hearing loss, short stature; other symptoms varied. According to the results of muscle biopsy, the phenomenon of «ragged red fibers» was found in two patients. The diagnosis was confirmed by molecular genetic examination. Conclusion. Мitochondrial diseases with the same mutation are characterized by significant variability of clinical symptoms. The identification of clinician traits characteristic of a group of mitochondrial diseases should alert the doctors to this pathology.