A boy with the intermediate variant of branched-chain ketoaciduria was studied. Treatment with an amino acid mixture was discontinued at the age of 7.5 years. Reintroduction of normal protein-containing foods precipitated the biochemical abnormalities, characteristic of MSUD, which were relieved by 10 mg thiamine/day. Adaptation to this regiment developed and thiamine intake was increased to 100 mg/day, later to 1 000 mg/day. The patient developed well and had no attacks of ketoacidosis. 1-14C-leucine degradation in intact fibroblasts was 15% of controls and did not increase upon addition of thiamine to the incubation medium.