The clinical problems, dietary management and biochemical monitoring over a 40-year period of the longest-surviving patient with maple syrup urine disease are described. Her case illustrates that a good outcome can be obtained with early diagnosis and institution of a diet restricted in branched-chain amino acids. Changes in dietary supplementation have benefited her in terms of nutrition and quality of life. Consistently high blood concentrations of branched-chain amino acids have not been associated with neuropsychometric decline.