A healthy 1.5-year-old girl went to sleep one night after vomiting. In the morning, the parents found the girl unconscious in bed and took her to the emergency room. The girl was 76.3-cm tall and weighed 8.4 kg with a body temperature of 35.2 C and blood pressure of 77/47 mmHg. She did not open her eyes in response to speech, and the results of simple blood glucose test were below the detectable limit. She regained consciousness following administration of 20 % blood glucose, but experienced tonic convulsions and rolling eyes for several minutes. The convulsions were controlled with phenobarbital. Intravenous fluids were started. The patient was lucid after 3 h, and her motor functions improved after 6 h to the point where she could walk on her own. Blood gas analysis at the time the patient was hospitalized showed pH 7.240, pCO2 41.5 mmHg, base excess -9.2, and HCO3 -17.2 mmol, indicating metabolic acidosis. Other abnormal results were lactic acid, 35 mg/dL; NH3, 118 lg/dL; AST, 143 U/L; and ALT, 46 U/L. Urine specific gravity was 1.04 with a urine ketone level of 3?. Organic acid test was performed on suspicion of congenital metabolic anomaly. The results showed a pronounced increase in methylmalonate excretion with increased methylcitrate and 3-OH-propionate excretion, leading to a diagnosis of MMA. Because the approximate urine methylmalonic acid level at onset was 658.4 lM/mmol creatinine, high-dose oral administration of Vitamin B12 (10 mg of cobamamide) was started, resulting in a decrease to 213.0 lM/mmol creatinine on day 7 of treatment. HPLC quantification of succinyl-CoA production upon reaction of the patient’s lymphocyte lysates with methylmalonyl-CoA and adenosylcobalamin revealed that the patient’s methylmalonyl-CoA activity level was 1.4 pmol succinyl-CoA/min/106 cells, which is approximately 5 % of the level seen in healthy controls (35.9 ± 16.4 pmol succinyl-CoA/min/106 cells). The patient has now been on high-dose oral administration of Vitamin B12 and carnitine therapy (900 mg of levocarnitine chloride) for 5 years. She has not experienced further attacks; her cognitive and motor development is normal.