A male infant developed progressive neuromuscular disease, hypertrophic cardiomyopathy and brain atrophy since the birth. Increased level of lactate with increased lactate/pyruvate ratio suggested a disturbance in the mitochondrial energy metabolism. The activities of respiratory chain complexes III, IV and II + III, of pyruvate dehydrogenase complex and of citrate synthase in isolated muscle mitochondria were low in comparison with controls, with parallel decrease in the content of protein amount of respiratory chain complexes III and IV. No large scale deletions of mitochondrial DNA (mtDNA) and mtDNA point mutations A3243G, A8344G or T8993G indicating syndromes MELAS, MERRF or NARP were detected. The boy died at the age of 7 weeks. The autopsy revealed typical changes of mitochondrial cardiomyopathy-marked myocardial hypertrophy with muscle pallor, histological finding of diffuse fine granularity of the cytoplasm in the perinuclear regions, and ultrastructural findings of mitochondrial hyperplasia, enlargement (megamitochondria) and abnormal shape.