We report a 5-month-old boy with recurrent vomiting, lethargy, and poor weight gain. He had profound metabolic acidosis and nonketotic dicarboxylic aciduria. The serum and muscle carnitine levels were significantly low (60% and 10% of the control means, respectively), suggesting that the patient had a systemic carnitine deficiency syndrome. The patient showed apparent clinical improvement on oral carnitine administration. A quadriceps muscle biopsy revealed a slight increase in intrafiber lipid droplets and mild accumulation of glycogen in the subsarcolemmal portion. An anaerobic glycolysis in vitro study showed a block after glucose-1-phosphate and before glucose-6-phosphate. Direct measurement of individual glycolytic enzymes in muscle of the patient demonstrated a marked decrease in phosphoglucomutase (PGM) activity (13% of the control mean). The specific defect of PGM activity in this patient suggests that the block in the anaerobic glycolytic pathway is the primary abnormality. PGM deficiency can be added as a newly recognized cause of secondary systemic carnitine deficiency syndromes.