A four-year-old wire-haired dachshund developed progressive neurological signs of ataxia, intention tremor and finally dysuria. Two years later, histopathology showed that neurons throughout the brain and spinal cord were distended with lipopigment which was also present in macrophages. Ultrastructurally, the pigment in the neurons occurred predominantly as electron-dense membranous whorls and stacks. There were a few vacuolated macrophages in the meninges. Hepatocytes were highly vacuolated and electron microscopy suggested that they were empty membrane-bound vesicles. The disease was diagnosed as mucopolysaccharidosis IIIA because of its similarity to other biochemically confirmed cases in the same breed and in a New Zealand huntaway dog. Additional lesions included calcium oxalate uroliths, severe secondary calcification of tissues including the brain and storage deposits in some neurons, and lesions which may have been associated with high levels of the substrate, heparan sulphate.