A 14-month-old girl with a virtual arrest in development at a 3-month stage was found to have methylmalonic acidemia. She was unresponsive to large doses of B12 or of dimethylbenzimadazole cobamide. Dietary therapy was devised to restrict protein containing isoleucine, threonine, valine, and methionine to the amounts required for growth. She had a dramatic response both clinically and chemically. Catch-up somatic growth was matched by growth in head size and intellectual development. These observations indicate that marked improvement in brain function may occur during the treatment of metabolic disease, even well after a year of age.