The main function of carnitine is the transport of fatty acids across the inner mitochondrial membrane to the side of beta-oxidation. In healthy subjects no carnitine deficiency occurs. There are many inborn errors with carnitine deficiency as a primary genetic defect or secondary to other familial disorders of metabolism. Furthermore some acquired diseases are associated with secondary carnitine deficiency. Myopathic and systemic forms of carnitine deficiency have been described. Most of the carnitine deficiency syndromes leading frequently to sudden death without therapy, are treatable with L-carnitine. A beneficial influence of L-carnitine to certain hyperlipoproteinemias, hyperlipidaemic diabetes mellitus and other diseases has been reported too.