Clinical manifestations, biochemistry and treatment of three patients with the inherited alteration of the so called maple syrup urine disease are presented. Diagnosis was based upon both detection of high concentrations of leucine, isoleucine, valine and their corresponding alpha-ketoacids in plasma and urine, and existence of a peculiar odor as well as an intense, progressive, neurological alteration. Considering the early onset of clinical symptoms, low protein tolerance and very reduced rate of leucine decarboxylation in cultured fibroblasts, the three patients exhibited grade I, classical type of this disease. None of them responded favorably to suprapharmacological doses of thiamine. Treatment of the patients is discussed and compared with that recommended in the literature.