The clinical hallmarks of muscle phosphorylase deficiency (McArdle's disease) are muscle cramps and exercise intolerance, contracture following ischemic work, and episodic myoglobinuria. We have encountered a patient with all these symptoms who proved to lack muscle phosphofructokinase (PFK) rather than phosphorylase. This paper describes the results of biochemical and immunological characterization of the metabolic abnormality. Phosphofructokinase deficiency was first described in a detailed biochemical study by Tarui et al,1 the only previous report. Phosphofructokinase catalyzes the conversion of fructose-6-phosphate to fructose1, 6-diphosphate (Fig 1). This enzymatic step is one of the main rate-limiting steps of glycolysis,2 and there is evidence that the regulation of glycolysis by aerobic metabolism (the Pasteur effect) is mediated by factors regulating the activity of PFK.3-5 Since glycogen accumulates in excessive amounts in muscle, PFK deficiency can be regarded as a fourth type of muscle glycogenosis, along with deficiencies of phosphorylase, amylo-1, 6-glucosidase