In vivo phosphorus magnetic resonance spectroscopy was used to evaluate the changes in muscle bioenergetics in a patient with a partial glycolytic block. Phosphoglycerate mutase-deficient muscle showed the following evidence: Abnormal accumulation of sugar phosphates does occur, even when 6% enzyme activity is present. The elimination of sugar phosphates was faster than in complete glycolytic blocks. Mild intracellular acidosis occurred during ischemic exercise. The energy state was slightly low at rest but not during exercise. Postexercise recovery was mildly slowed. These findings suggest that phosphorus magnetic resonance spectroscopy can detect partial defects, as well as full glycolytic blocks, in muscle metabolism.