Metabolic decompensation of maple syrup urine disease in a 3.5‐year‐old boy was monitored by means of proton magnetic resonance imaging and spectroscopy. In the acute stage, imaging showed diffuse cerebral edema. Proton spectra showed an elevation of lactate and a previously unassigned resonance at 1 ppm. This peak disappeared with normalization of branched‐chain amino acids and oxoacids in the plasma and cerebrospinal fluid. In vitro spectroscopy of these metabolites at 1.5 T confirmed the chemical shift position of their methyl resonances at 1 ppm. The duration of lactate elevation correlated with the presence of brain edema and coma. These results demonstrate that magnetic resonance imaging can monitor therapeutic effects, and suggest that proton spectroscopy can detect cerebral accumulation of branched‐chain amino acids and oxoacids in maple syrup urine disease.