Forty randomly selected dogs, arranged into three groups according to age, were examined light and electron microscopically. Lafora-like inclusion bodies were found in all cases of aged dogs without neurological signs. They appeared as PAS positive, round bodies measuring up to 15 mu in diameter. Typically they had central cores and radiating peripheral lines. Electronmicroscopically, the inclusions consisted of irregularly clustered, short branching filaments measuring about 80--120 A in diameter or of electron-dense homogeneous or granular central cores with filamentous peripheral areas and located both in the perikarya and neuronal processes. The inclusions were disseminated all over the brain and spinal cord. The thalamic nuclei, a circumscribed area dorsal to the aqueduct, and the molecular layer of the superior colliculus were the most severely affected. The frequency of the inclusions showed age-dependency; the inclusions were not found in dogs younger than 2 years, but were extensive in all dogs of more than 8 years of age. The occurrence of the inclusions may therefore represent an age-dependent phenomenon. The relationship between the present findings and Lafora's disease is discussed.