Clinical and biological criteria of myopathies associated with carnitine deficiency allow to distinguish a muscular and a systemic form of the condition. In this report, the results of clinical, pathological and electrophysiological data obtained from a patient with carnitine deficiency-linked myopathy are described. The patient was a 23-year-old girl who was previously known to suffer from muscle weakness when suddenly acidosis associated with a severe drop in plasma carnitine appeared. In addition there were hypermetabolic symptoms similar to those described in Luft's syndrome. Biopsy from the quadriceps femoris muscle before treatment revealed that all type I fibers were either hypotrophic or atrophic. They showed lipid overloading manifested by triglyceride droplets adjacent to the mitochondrial membrane. Furthermore, the level of soluble muscle carnitine was 83 p. 100 less than in controls and membrane linked muscle carnitine was also 73.5 p. 100 less than in controls. The patient rapidly recovered after the initiation of daily treatment with 4.40 g carnitine chlorhydrate associated with 50 g Lipogram 20. Nine months later, lipid overloading completely disappeared and the level of plasma carnitine returned to near normal whereas the level of both soluble and linked carnitine remained very low. To provide more information on the origin of the myopathy (myogenic, neurogenic or humoral) we carried out an electrophysiological investigation of cultured skeletal muscle cells from the patient and from biopsies of patients not known to be suffering from myopathy. The electrophysiological data showed that the patient myotubes were less polarized than myotubes from control patients. Furthermore, the amplitude of the action potential was smaller than the amplitude of the action potential measured in control cells. Daily addition of 50 microM carnitine chlorhydrate to the cultured myotubes induced a recovery of the action potential amplitude. Taken together these results indicate that the carnitine deficiency reported here was probably of systemic origin in addition to a myogenic component. Muscle deficiency could be either linked to an alteration in the carnitine pathway or to overconsumption of carnitine by muscle. This latter point is discussed.