An increasing number of cases of myopathy due to disordered lipid metabolism have recently been recognised and these appear to fall into two contrasting clinical and biochemical categories. Some patients present with steadily prograssive proximal weakness which sometimes responds to steroid therapy and which is due to carnitine deficiency in skeletal muscle. In the second category, patients usually present with muscle cramps on exertion then followed by myoglobinuria without established proximal myopathy and their symptoms are associated with deficiency of carnitine palmityl transferase in skeletal muscle. In this paper we report a patient who has well-documented carnitine palmityl transferase deficiency, whose symptoms were triggered by violent exercise after fasting and in whom there was variable histochemical and ultrastructural evidence of lipid accumulation in muscle biopsy samples. Development of the patient's symptoms was suppressed by a high carbohydrate diet.