A third case of phosphoglycerate mutase (PGAM) deficiency, a metabolic myopathy involving terminal glycolysis, was identified in a 24-year-old black man with episodic, exercise-induced myoglobinuria since age 13. To better understand the physiologic consequences of PGAM deficiency, incremental exercise testing was performed. Results were compared with those of two patients having myophosphorylase deficiency and five normals. In contrast to the patients with phosphorylase deficiency, the PGAM-deficient patient achieved near-normal levels of maximal exercise and produced a normal peak lactate after exercise. The mechanisms underlying the asymptomatic performance of such strenuous exercise in this case are uncertain, but the data suggest that unidentified factors are operative in precipitating attacks of myoglobinuria in patients with some metabolic myopathies. Despite similar clinical histories, patients with different glycolytic enzyme deficiencies can have striking differences in exercise tolerance.