Methylmalonic acidemia results from decreased activity of methylalonyl-CoA mutase, an enzyme required for the catabolism of four amino acids. A cobalamin (vitamin B12) compound is required as coenzyme. Several inherited mutations of the mutase apoenzyme or of cobalamin coenzyme synthesis have been identified. Clinical disease is most commonly recognized as a severe protein intolerance state although a wide range of symptoms has been appreciated. Current therapy emphasizes strict dietary management and use of vitamin B12 in cobalamin responsive patients.