Maple syrup urine disease (MSUD) is an inherited deficiency of branched chain α-ketoacid dehydrogenase (BCKDH) activity impairing the degradation of the branched chain amino acids valine,leucine,and isoleucine. Classic MSUD may lead to severe neonatal encephalopathy including coma and impaired cognitive outcome in later life. Early start of dietary treatment and careful metabolic control may improve the outcome of patients with classic MSUD. The aim of this study was to investigate the impact of long-term metabolic control assessed by plasma leucine levels on cognitive outcome in patients with classic MSUD. Plasma leucine levels of 24 patients were obtained retrospectively for the first 6 y of life and yearly medians of mean plasma leucine levels were calculated. At the age of 6 y,IQ tests were performed. Yearly medians of mean plasma leucine levels yielded three homogeneous clusters (low,intermediate,high). Patients of the low cluster showed statistically significant higher IQ scores compared with those of those of intermediate and high clusters. Long-term plasma leucine levels are associated with impaired cognitive outcome in patients with classic MSUD. To achieve the best possible intellectual outcome for affected individuals,we recommend that in infants and preschool children the target range for plasma leucine should not exceed 200 μmol/L. Maple syrup urine disease (MSUD) is an inherited deficiency of branched chain alpha-ketoacid dehydrogenase (BCKDH) activity impairing the degradation of the branched chain amino acids valine, leucine, and isoleucine. Classic MSUD may lead to severe neonatal encephalopathy including coma and impaired Early outcome of dietary treatment and careful metabolic control may improve the outcome of patients with classic MSUD. The aim of this study was to investigate the impact of long-term metabolic control assessed by plasma leucine levels on cognitive outcome in Patients with classic MSUD. Plasma leucine levels of 24 patients were retrieved retrospectively for the first 6 y of life and medium medians of mean plasma leucine levels were calculated. At the age of 6 y, IQ tests were performed. levels yielded three homogeneous clusters (low, intermediate, high). Patients of the low cluster had statistically significant higher IQ scores compared with those of intermediate of high clusters. Long-term plasma leucine levels are associated with impaired cognitive outcome in patients with classic MSUD. To achieve the best possible intellectual outcome for affected individuals, we recommend that in infants and preschool children the target range for plasma leucine should not exceed 200 μmol / L.