Maple syrup urine disease (MSUD) is a defect in the catabolism of the branched-chain amino acids; leucine, isoleucine, and valine. Affected patients may also develop hyperammonaemia of unknown etiology. This report describes a four-year-old girl with MSUD, who presented with decompensated hyperleucinaemia with hyperammonaemia. The oral administration of the N-acetylglutamate analogue, N-carbamylglutamate (NCG), 200 mg/kg/day as a loading dose, and 100 mg/kg/day as a maintenance dose, in combination with standard therapy resulted in a significant decrease of plasma ammonia levels. This observation suggests that NCG may be an important adjunct to standard therapy in the management of decompensated MSUD patients with high leucine and ammonia levels. Supportive evidence from either randomized controlled trials or a large prospective cohort study is needed to confirm this interesting finding.