Mitochondrial mutations are associated with a wide range of disorders (Kearns-Sayre and chronic progressive external ophthalmoplegia syndromes, Myoclonic epilepsy and ragged-red fibre disease, Mitoehondrial encephalomyopathy, lactic acidosis, and stroke-like episodes, Leighs disease ancl cerebellar ataxia plus pigmentary retinopathy syndromes), which is inherited maternally. A-to-G mutation at nuclcotide position 3243 was originally identified in MEI.AS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) and accounted for about 80% of the MELAS cases, Recently, this mutation was reported in maternally inherited NIDDM patients. It was also repoded that approximatedly 1% of diabetic patients have this mutation. We performed the molecular genetic analysis of mtDNA in one female insulin dependent diabetic patient with MELAS syndrome and her family members, and also confirmed the A-to-G mutation at nucleotide 3243 of the mtRNA Leu(UUR) gene in their family members.