Canine neuroaxonal dystrophy, a newly recognized familial disorder in Rottweiler dogs, is characterized by progressive sensory ataxia. Two of four dogs studied clinically were autopsied and the cerebellum was mildly atrophic. Massive numbers of axonal spheroids were present in many regions of the neuraxis but were most prominent in the dorsal horn of the spinal cord and the nuclei gracilis and cuneatus. Ultrastructurally, spheroids appeared to be swellings of distal axons which were filled with accumulations of smooth membrane-bound vesicles, membranous lamellae, dense bodies, and other organelles. Neuropathological changes were similar to those identified in human neuroaxonal dystrophy.