Glutaric aciduria type I is due to an impaired glutaryl-CoA-dehydrogenase with an increased urinary excretion of glutaric and 3-OH glutaric acid. Typically, the clinical course until the sixth month or even 3rd year of life is symptom free, and only later an encephalopathic crisis develops. The only symptom of our 4 patients was macrocephaly (head circumference greater than 97. percentile) in early infancy. 3 of them suffered from an encephalopathic crisis at 8 months to 3 years of age; during that time they lost already established abilities as sitting, walking and speaking, and developed choereoathetotic movements. One child aged 15 months was normal beside it's macrocephalus. All children were treated with a diet low in lysine (80 mg/kg BW/day), tryptophane (21 mg/kg BW/day), and by supplementation of L-carnitine (200 mg/kg BW/day) and riboflavine (200 mg/day) and the motorically disturbed children received Lioresal 1 mg/kg BW/day. The effect of this treatment cannot be evaluated so far, but there is evidence that the dietetic therapy together with carnitine supplementation may prevent further deterioration in affected, or an encephalopathic crisis in unaffected patients. Therefore we suggest to investigate organic acids in urine in every child or infant with macrocephalus to exclude glutaric aciduria type I.