A case of maple syrup urine disease is presented. The patient lived for 20 months, the longest survival so far reported. There were increased amounts of leucine, isoleucine and valine in plasma and urine, indicating a block in the metabolic degradation of these amino acids. There was an accumulation in the urine of the respective keto acids, but not of the aldehydes or of the simple acids. This locates the block at a step common to all three amino acids, that of oxidative-decarboxylation. The alpha-hydroxy-acid derivatives of these amino acids are probably present, but the evidence is still incomplete. Animal experiments indicate that oxidative decarboxylation of the branched-chain amino acids is a metabolic step which is found generally in tissues, including leukocytes but probably not erythrocytes.