Four patients with classical maple syrup urine disease were treated for up to 5885 days per patient with a relaxed protocol allowing branched-chain amino acid levels in plasma to rise about 5 times the normal mean value. The patients have had satisfactory development and lifestyle. They spent 318 days in hospital during 19,937 aggregate treatment days. Plasma levels of leucine and the corresponding 2-oxo acid were shown to be elevated disproportionately relative to the other branched-chain metabolites. Levels of isoleucine and valine were lower than those of leucine apparently because of runout into alternative metabolite pools, namely the R metabolites for isoleucine and the hydroxyacid for valine. The chronic accumulation of branched-chain 2-oxo acid(s) in our patients was associated with chronic dysmyelinating changes in CNS visible by imaging. Another patient with a thiamine-responsive variant of maple syrup urine disease had five acute crises incurring 29 days in hospital in a total of 6910 treatment days. However, she did not have chronic metabolic dyshomeostasis (her average plasma amino acid values were normal) and she had no evidence of dysmyelination. A relaxed treatment protocol for patients with maple syrup urine disease may benefit them in quality of life, but it apparently exacts a cost in metabolic control and CNS pathology.