Glycogen storage disease IIIa is an inherited metabolic disorder affecting dogs. Glycogen is the primary carbohydrate used by the body for energy. Affected dogs do not have sufficient enzymes to break down glycogen causing accumulation of glycogen in the liver and muscle. Though symptoms are mild in the first year of life and disease progression is variable, affected dogs typically present between one and two years of age with exercise intolerance, lethargy, vomiting, enlarged liver, and in some dogs, collapse with exercise secondary to low blood sugar. Progressive liver and muscle damage occur with age as glycogen continues to accumulate in cells.