Primary/systemic carnitine deficiency (PCD) is a disorder of fatty acid oxidation caused by defective carnitine transport. Patients may present as infants with nonketotic hypoglycemia, hypotonia, Reye syndrome or sudden infant death or later in life with cardiomyopathy (characteristically dilated) or muscle weakness. Different types of presentation have been observed within an individual family. Diet may contribute to the pathogenesis of this disorder. Delayed diagnosis has been reported as common. PCD can be identified by MS/MS based newborn screening and positive newborn screening results have also occurred in infants of mothers affected with PCD even though the mother has had mild or no symptoms. Patients respond promptly to carnitine supplementation, with correction of metabolic abnormalities, and improvement in skeletal myopathy and cardiomyopathy reported. However, developmental delay due to hypoglycemia that occurred prior to treatment usually persists.