The authors investigated whether carriers of single myophosphorylase gene (PYGM) mutations have symptoms of McArdle disease. They studied the oxidative capacity and lactate responses to maximal cycle exercise in eight patients with McArdle disease, seven single PYGM mutation carriers, and 11 healthy subjects. Heterozygotes had maximal oxidative capacity and peak lactate responses identical to control subjects. Thus, carriers of single PYGM mutations are not prone to develop symptoms of McArdle disease.