The authors report a case of concurrent moyamoya disease and glycogen storage disease Type 1a that was successfully managed with bypass surgery. This 7-year-old Japanese girl, diagnosed with glycogen storage disease Type 1a at the age of 2 years, presented with repeated transient ischemic attacks. Cerebral angiography revealed severe stenosis at the terminal portions of the bilateral internal carotid arteries, with typical moyamoya vessels. The patient underwent superficial temporal artery–middle cerebral artery anastomosis and encephalomyosynangiosis bilaterally, in 2 staged procedures at an interval of 4 months. Despite perioperative administration of glucose, hypoglycemia and metabolic acidosis occurred after both surgeries. The symptoms were milder after the second surgery, in which an increased dose of glucose was used. The patient tolerated the perioperative conditions well under intensified medical treatment, and no further ischemic symptoms occurred.