A fatal case of systemic carnitine deficiency is reported. The patient suffered from slowly progressive muscle weakness since early childhood. After the age of 17 years her weakness progressed more rapidly until her death at the age of 20. A pregnancy during the last year of the patient's life was followed by rapid deterioration in her condition. An episode of renal insufficiency occurred at the age of 17 years and hepatomegaly, increased BSP dye retention and intermittent ketoacidosis were present during the last month of her life. Biopsy and autopsy specimens of muscle showed a lipid storage myopathy. Type 1 fibers were selectively severely affected, and many Type 1 fibers were atrophic. Abundant large mitochondria, some also containing abnormal inclusions, were also present in the muscle fibers. At autopsy there was marked accumulation of sudanophilic lipid deposits in all hepatocytes, in the renal tubular epithelial cells, and a patchy increase of lipid material was found in the myocardial fibers. There was marked carnitine deficiency in the patient's liver as well as muscel, while the carnitine palmityltransferase activities in these tissues were abnormally high. The basic metabolic abnormality is assumed to be a defect in carnitine biosynthesis.