BACKGROUND A heteroplasmic A3243G point mutation in tRNALeu(UUR) gene of mitochondrial DNA (mtDNA) is found in patients with MELAS syndrome (Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes), less frequently in patients with other dominating clinical features, such as deafness, diabetes mellitus type 2, hypertrophic cardiomyopathy, renal problems or inborn development defects. Present report describes histochemical, enzymatic and molecular biology studies of the family with clinical variant of meals syndrome. RESULTS A 45-year-old woman with progressive muscle weakness, external ophtalmoplegia, perceptive deafness, ischemic heart disease, diabetes mellitus type 2 and hyperlactacidemia was metabolically investigated because the multiorgan problems indicated mitochondrial origin of the disease. Muscle biopsy revealed pronounced myopathic changes, ragged red fibers and decreased activity of respiratory chain enzymes - succinate cytochrome c reductase (< 5% control) and cytochrome c oxidase (< 10% control). Restriction fragment analysis of mtDNA from muscle, blood and hair follicles detected heteroplasmic A -> G mutation in the position 3243 of the tRNALeu(UUR) gene, which was more pronounced in muscle (28% of total mtDNA) than in blood (12%) or in hair follicles (10%). No mutation was found in blood and hair follicles of patient's mother and daughter. CONCLUSIONS Diagnostics of mitochondrial diseases require close collaboration of clinicians with specialised laboratories. Treatment of mitochondrial disorders is only symptomatic, however, early diagnosis of the molecular defect is important for genetic counselling.