A description is given of the macroscopic and microicopic picture of a rare entity of glycogenic cardiomegaly in a dog. Clinically it was diagnosed as a combination of tonsillitis, oesophageal dilatation and myocarditis. The characteristic symptoms were increasing difficulty in swallowing, sporadic vomiting, copious amount of mucus in the pharynx, unthriftiness and poor stale of nutrition. The histochemical changes were characteristic, indicated a process of glycogenosis and conformed to the classic form of Pompe's disease. Beside the enormously hypertrophied heart, the process successively involved the brain, liver, lungs, oesophagus and kidneys. An interesting diagnostic pattern of glycogenosis was revealed in capillaries and in all arterial media. The literature on glycogen storage disease is briefly reviewed and critically discussed with reference to the case presented.