Methylmalonic acidemia is an autosomal recessive disorder, caused by complete or partial deficiency of the enzyme methylmalonyl-CoA mutase, a defect in the transport or synthesis of its cofactor, adenosyl-cobalamin, or deficiency of the enzyme methylmalonyl-CoA epimerase. Onset of this condition is usually in early infancy, and it varies from mild to life threatening. Its manifestations include: lethargy, failure to thrive, recurrent vomiting, dehydration, respiratory distress, muscle hypotonia, hepatomegaly and coma. Diagnosis is based on analysis of organic acids in plasma and/or urine by gas-liquid chromatography and mass spectrometry. Also, uncovering this condition through neonatal screening for propionylcarnitine and/or increased propionylcarnitine-to-acetylcarnitine ratio in dried blood spots by tandem mass spectrometry (MS/MS) has become common. Treatment includes a protein-restricted diet, increased calories via high glucose-containing fluids and insulin to arrest catabolism; and monitoring serum electrolytes and ammonia, venous or arterial blood gases, and urine output. Patients with this type are responsive to vitamin B12.