We describe two brothers, 25 and 19 years-old, with muscle pain and decreased strenght after prolonged exercise; these symptoms are worsened by cold whether of fasting. One of the patients developed recurrent myoglobinuria and had one episode of renal failure. Laboratory investigations were normal between the crises, but during myoglobinuria, serum creatinekinase activity increased 100 times. Electromyography was suggestive of denervation. Muscle biopsy showed increased lipid droplets by the "oil red O" stain and increased activity of succinic dehydrogenase histochemical reaction. Lactate production during ischemia was normal. Biochemical analysis showed decreased carnitine-palmityl-transferase activity in muscle (7.23 and 10.58 nmoles/min/gr; normal range 66.7 ± 17.3), with normal values for carnitine-octanoyl-transferase and carnitine-acetil-transferase. The metabolic pathway of fatty acid utilization as an energy source for muscle during exercise in normal and in pathological conditions is discussed.