Dietary therapy in a girl with Vitamin B12 unresponsive methylmalonic acidemia was described. A diet containing less than 1.2g per kg body weight of protein was well tolerated, whereas 1.5g per kg body weight resulted in ketoacidosis. Amino acid mixture devoid of toxic amino acids was effective to prevent protein malnutrition while daily protein intake was restricted to 0.6g per kg body weight. Caloric intake should be sufficient to prevent a rise in MMA level. Successful treatment was achieved on the regimen containing 1.Og per kg body weight of protein, 1.0g per kg of the amino acid mixture and 70 to 80 kcal per kg of caloric intake.