A patient is described who was admitted with a condition similar to the Reye syndrome at the age of 9 months. Hypoglycemia, hyperammonemia, hepatomegaly, and lethargy were present. The plasma concentrations of free and acylcarnitine were extremely low and the urine contained excessive amounts of dicarboxylic acids. Extensive biochemical and histological investigations of biopsied liver and muscle led to the diagnosis of systemic carnitine deficiency. The patient was put on oral carnitine treatment, upon which he remained clinically well. A prolonged fasting test during this treatment gave abnormal results: there was no ketonemia, but an increase of omega-oxidation of fatty acids. In spite of the treatment the liver and muscle carnitine content remained below normal.