No effective treatment for McArdle disease exists.We report a Japanese patient with McArdle disease who was treated with vitamin B(6) supplementation (60-90 mg/day). After treatment, increased muscle phosphorylase activity was confirmed by follow-up muscle biopsy (3.8 times higher than pretreatment levels). Increased lactate levels were seen on the forearm exercise test, and regular work activities could be resumed. Vitamin B(6) supplementation can enhance residual phosphorylase activity and improve insufficient anaerobic glycolysis of skeletal muscle.