The erythrocyte glycogen concentration has been determined in 63 normal persons and 18 patients with glycogen storage disease. Erythrocyte glycogen from normal patients and from those with glycogenosis has been isolated and characterized by beta-amylase degradation and the iodine spectrum. By these techniques patients with Type III glycogenosis could be distinguished both by the concentration of glycogen in the erythrocyte and characterization of the glycogen. One patient with amylo-1,4→1,6-transglucosidase deficiency (Type IV) was found to have a normal concentration of glycogen in the erythrocytes but the glycogen from the cellular elements of the blood was shown to be abnormal, being amylopectin-like in structure. The concentration of glycogen in the erythrocytes of Types I and II glycogenosis was found to be within the normal range. The etiologic diagnosis can be made in approximately 50% of the patients with glycogenosis by assay of the concentration and characterization of erythrocyte glycogen, thereby obviating the necessity for biopsy. The erythrocyte glycogen in Type III glycogenosis offers another parameter which may be followed in evaluating the response of the patient to therapy.