The clinical manifestations of mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS syndrome) are nonspecific and can easily be misdiagnosed. Magnetic resonance spectroscopy (MRS)-based detection of lactate in the brain has been found to be of diagnostic help in MELAS syndrome, however, the issue of whether MRS features vary by stage remains unresolved. We assessed the causative mutation and radiological features of a family of MELAS. Four of the family members harbored the A3243G mutation, probably of maternal inheritance. However, the clinical phenotypic expression was different in these patients. MRS showed a lactate peak, decreased N-acetylaspartate, choline, and creatine, which became more pronounced with progression of the disease, demonstrating that brain-MRS-based detection of lactate may be a suitable way to monitor the progression and treatment of MELAS.