A 23-year-old woman with McArdle's disease performed mild leg exercise on a bicycle ergometer. Saline or 10% glucose solution was infused throughout exercise. After exercise with saline infusion, her plasma concentrations of ammonia, hypoxanthine and creatine kinase increased greatly. Conversely, after exercise with glucose infusion, there were no appreciable changes in these plasma substances. In addition, she noticed that glucose infusion relieved her from muscle symptoms during exercise. These findings suggest that glucose infusion to patients with McArdle's disease ameliorates excessive ATP degradation in exercising muscles.