Aim: To investigate the in vivo skeletal muscle metabolism in patients with β-enolase deficiency (GSDXIII) during exercise, and the effect of glucose infusion. Methods: Three patients with GSDXIII and 10 healthy controls performed a nonischemic handgrip test as well as an incremental cycle ergometer test measuring maximal oxidative consumption (VO_2max) and a 1-hour submaximal cycle test at an intensity of 65% to 75% of VO_2max. The patients repeated the submaximal exercise after 2 days, where they received a 10% iv-glucose supplementation. Results: Patients had lower VO_2max than healthy controls, and two of three patients had to stop prematurely during the intended 1-hour submaximal exercise test. During nonischemic forearm test, all patients were able to produce lactate in normal amounts. Glucose infusion had no effect on patients' exercise capacity. Conclusions: Patients with GSDXIII experience exercise intolerance and episodes of myoglobinuria, even to the point of needing renal dialysis, but still retain an almost normal anaerobic metabolic response to submaximal intensity exercise. In accordance with this, glucose supplementation did not improve exercise capacity. The findings show that GSDXIII, although causing episodic rhabdomyolysis, is one of the mildest metabolic myopathies affecting glycolysis.