BACKGROUND Phosphoglucomutase type 1 (PGM1) deficiency is a rare metabolic myopathy in which symptoms are provoked by exercise. OBJECTIVE Because the metabolic block is proximal to the entry of glucose into the glycolytic pathway, we hypothesized that iv glucose could improve the exercise intolerance experienced by the patient. METHODS This was an experimental intervention study. METHODS The study was conducted in an exercise laboratory. METHODS Subjects were a 37-year-old man with genetically and biochemically verified PGM1 deficiency and 6 healthy subjects. METHODS Cycle ergometer, peak and submaximal exercise (70% of peak oxygen consumption), and exercise with an iv glucose infusion tests were performed. METHODS Peak work capacity and substrate metabolism during submaximal exercise with and without an iv glucose infusion were measured. RESULTS Peak work capacity in the patient was normal, as were increases in plasma lactate during peak and submaximal exercise. However, the heart rate decreased 11 beats minute⁻¹, the peak work rate increased 12.5%, and exercise was rated as being easier with glucose infusion in the patient. These results were in contrast to those in the control group, in whom no improvements occurred. In addition, the patient tended to become hypoglycemic during submaximal exercise. CONCLUSIONS This report characterizes PGM1 deficiency as a mild metabolic myopathy that has dynamic exercise-related symptoms in common with McArdle disease but no second wind phenomenon, thus suggesting that the condition clinically resembles other partial enzymatic defects of glycolysis. However, with glucose infusion, the heart rate decreased 11 beats min⁻¹, the peak work rate increased 12.5%, and exercise was considered easier by the patient.