Maple syrup urine disease (MSUD) is an autosomal recessive disorder caused by defects in the mitochondrial multienzyme complex branched chain ketoacid dehydrogenase (BCKD; EC 1.2.4.4), responsible for the oxidative decarboxylation of the branched chain ketoacids (BCKA) derived from the branched chain amino acids (BCAA), leucine, valine and isoleucine. Deficiency of the enzyme results in increased concentrations of the BCAA and BCKA in body cells and fluids. The treatment of the disease is aimed at keeping the concentration of BCAA below the toxic concentrations, primarily by dietary restriction of BCAA intake. The objective of this study was to determine the total BCAA requirements of patients with classical MSUD caused by marked deficiency of BCKD, using the indicator amino acid oxidation (IAAO) technique. Five MSUD patients from the MSUD clinic of The Hospital for Sick Children participated in the study. Each was randomly assigned to different intakes of BCAA mixture (0, 20, 30, 50, 60, 70, 90, 110, 130 mg^.kg^-1. d^-1), in which the relative proportion of BCAA were the same as those in egg protein. Total BCAA requirement was determined by measuring the oxidation of L-[1-¹³C]phenylalanine to ¹³CO₂ (F¹³CO₂). The mean total BCAA requirement was estimated using a two-phase linear regression crossover analysis, which showed that the mean total BCAA requirement was 45 mg^.kg^-1. d^-1, with the safe level of intake (upper 95% confidence interval) at 62 mg^.kg^-1. d^-1. This is the first time BCAA requirements in patients with MSUD have been determined directly.