The records of 11 dogs with evidence of meningoencephalomyelitis of unknown origin were reviewed. Two of them had had a focal form of the disease and the other nine a disseminated form. The forebrain was involved in five of the nine dogs with disseminated disease, the brainstem in all nine and the cerebellum in one. They had been treated with courses of cytosine arabinoside every three weeks and immunosuppressive doses of prednisolone. Their response to the treatment, in terms of quality of life, was judged by their owners and referring veterinarians to have been excellent in five, good in five and poor in one; their survival times ranged from 78 days to more than 603 days. The cumulative probability of survival at two years was 58.4 per cent. No signs of myelosuppression or other side effects associated with cytosine arabinoside were observed.