We report four patients with methylmalonic acidemia who developed acute extrapyramidal disease after metabolic decompensation. The neurologic findings resulted from bilateral destruction of the globus pallidus with variable involvement of the internal capsules. This complication was unrelated to a specific gene defect responsible for methylmalonic acidemia or to cyanocobalamin administration. These lesions constitute a "metabolic stroke," probably because of the accumulation of toxic organic acid metabolites, because they cannot be accounted for by hypoxemia or vascular insufficiency.