Glutaric aciduria type I is an autosomal recessive metabolic disease (1 case/30,000) characterized by a progressive dystonic-diakinetic syndrome in children. Pathologic examination reveals striatal degeneration of the caudate and putamen nucleus and biochemical analysis shows glutaryl CoA dehydrogenase deficiency. Values of glutaric and -hydroxyglutaric acids in urine are usually increased. Currently, the disease is considered untreatable since there are usually irreversible lesions in the central nervous system at diagnosis. However, treatment can be provided to pre-symptomatic children and usually to the siblings of patients with this diagnosis. We present the case of a 23-month-old boy, with macrocephaly and minimal neurologic manifestations at diagnosis, which were attributed to his semivegetarian diet. A dietary regimen and vitamin supplementation halted and even improved symptomatic progression of the disease. We conclude that amino and organic acids in urine should be investigated in all children with progressive macrocephaly of unknown etiology to rule out glutaric aciduria type I.