The contemporary conceptualization of etiology, pathogenesis, clinical features, and diagnostic criteria of Kearns — Sayre syndrome, one of the forms of mitochondrial cardiomyopathy, is presented. The case record of a patient with the classic variant of Kearns—Sayre syndrome is presented. The timely diagnosis importance in progressive lesions of the cardiac conduction system is discussed and the role of molecular genetic analysis in the diagnosis verification is emphasized.