Methylmalonic acidemia, also known as methylmalonic aciduria, is an autosomal recessive disorder. The main clinical manifestation of this disease is its early onset. It usually develops in newborns or early infants, and often presents with lethargy, poor growth, recurrent vomiting, dehydration, respiratory distress, hypotonia, and in some children, mental retardation, hepatomegaly, coma, etc. The treatment of this disease is more effective with dietary therapy. Early on, protein intake is restricted and the intake of methylmalonic acid precursor amino acids is reduced. Levocarnitine and oral antibiotics may be effective. Some cases are effective with high-dose vitamin B12 supplementation, i.e., vitamin B12-dependent methylmalonic acidemia, which can be treated by first giving vitamin B12 for one week. If a significant effect occurs, the child can be maintained on oral administration for a long period of time, with the dose of vitamin B12 adjusted according to clinical and blood biochemistry.