We examined brain energy metabolism by phosphorus-31 magnetic resonance spectroscopy (31P-MRS) in the occipital cortex in a mother and a daughter with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) during photic stimulation. The peak area ratio of phosphocreatine markedly decreased during photic stimulation, and subsequently increased after the stimulation. This method, photic stimulation-31P-MRS, may be useful in assessing brain energy metabolism in neurological diseases.