A glycogen storage disease affecting primarily the skeletal muscle and, to a lesser degree, the cardiac muscle, spinal cord, and brain was diagnosed in a 10-year-old neutered Abyssinian cat with a 4-year history of paresis progressing to acute paralysis. Microscopically, these tissues contained inclusions that were pale basophilic in hematoxylin and eosin-stained slides, diastase resistant, periodic acid-Schiff positive, and blue-to-almost black with iodine stain. By transmission electron microscopy, the inclusions consisted of cytosolic, usually sharply demarcated, nonmembrane-bound deposits of finely granular and filamentous material. On the basis of the structural and histochemical staining characteristics, the inclusions were believed to be aggregates of abnormally stored, unbranched glycogen. A defect in glucose metabolism is suspected to be the underlying pathologic process, but an exact cause remains elusive.