We describe the pregnancy course and labor in a woman with myophosphorylase deficiency (McArdle鈥檚 disease or glycogen storage disease type V). McArdle鈥檚 disease is an uncommon hereditary disorder characterized by symptoms of muscular fatigue and pain, as well as by muscle cramps during heavy and continuos exercise. We feared that the increase in muscular effort during labor might complicate delivery, necessitating cesarean section.