The effect of a diet enriched with fats on the muscle performance of a patient with McArdle's syndrome was studied. The tolerance to physical activity was studied during exercise (ergometric examination) and by sustained abduction to 90° of the deltoid muscle, both after a three-day period on a normal diet and after a three-day period on a fat-rich diet. After the first period a woody spasm of the deltoid muscle was found which lasted several days. After a period on fat-rich diet the patient's physical fitness was increased and the recovery period after the acute load was shorter. Moreover, no induration of the deltoid muscle was observed after the sustained abduction to 90°. It is argued that maximal strength is not improved by a fat-rich diet but tolerance to submaximal loads is increased by such treatment and recovery from non-lactate-produced muscle discomfort is hastened.