Hepatic and muscle forms of carnitine palmitoyl transferase (CPT) deficiency have been described: the hepatic form is attributable to deficiency of CPT I, the muscle form to deficiency of CPT II. The muscle form of the disease usually becomes symptomatic in the second decade of life or later and is manifested as recurrent muscle fatigue, pain, and myoglobinuria triggered by metabolic challenges such as fasting and prolonged exercise, which normally require fatty acid oxidation as a major source of muscle energy production. Exertional muscle cramps are not a feature of the disorder, and patients are not subject to muscle injury during brief bouts of intense exercise or during the transition from rest to exercise, as is the case in uscle glycolytic defects. The metabolic defect promotes glycogen depletion, because the glycogen-sparing effect of fatty acid oxidation is lacking. Effective treatment consists of maintaining adequate muscle glycogen stores by eating a high-carbohydrate diet, having regular meals, and avoiding prolonged exercise. Prompt medical attention should be sought to reduce the risk of acute tubular necrosis should prolonged muscle pain or pigmenturia occur.