Pathological and biochemical studies were performed on a 9-month-old boxer dog with progressive neurological abnormality. Histological examination revealed marked neuronal storage throughout the central nervous system and histiocytic storage in the reticuloendothelial system. Ultrastructurally, the neuronal storage consisted of accumulation of concentric membranous inclusions and clusters of dense bodies. The biochemically unesterified cholesterol content was high in the liver and spleen. The brain showed increased levels of lactosylceramide and two gangliosides, GM3 and GM2. These findings indicate that this dog was affected with a heterogeneous lipid storage disease similar to the human Niemann-Pick type C disease.