A new case of neonatal 3-hydroxy-3-methylglutaric aciduria is described. 3-hydroxy-3-methylglutaryl CoA lyase activities in leukocytes demonstrated the patient's homozygosity and the heterozygous character of the parents and two other members of the family. Dietetic management with low fat high carbohydrate diet together with protein restriction and carnitine resulted in a good control of the metabolic acidosis, the hypoglycemia, and the physical and neurological development. Nevertheless, sudden death occurred at the age thirteen months without any previous apparent trouble and the necropsia showed neither signs of infection nor hepatic or cardiac derangement.