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A 10-year-old mixed-breed dog was presented with progressive neurological deficits suggestive of polyneuropathy. CT and MRI revealed no relevant abnormalities, whereas the CSF analysis showed a marked lymphocytic pleocytosis, immunocytochemically classified as T-cell lymphoma. Chemotherapy ensued with no improvement of clinical signs. Cerebrospinal fluid reanalysis still revealed a mild lymphocytic pleocytosis. MRI of the brain and spinal cord solely showed a reduced ventral subarachnoid and epidural space (Th11-L2) and mild enlargement of the central canal in this area. Postmortem examination revealed thickening of the spinal nerve roots and ganglia. Histologically, a marked intraspinal-subdural and intraneural, as well as intra-ganglionic, infiltration by large lymphocytes in the peripheral nervous system was visible. Immunohistochemically, the infiltrating lymphoblasts were CD3 positive, confirming T-cell origin. Neurolymphomatosis is a rarely described neoplastic disease in dogs. The present case highlights the importance of analysis of CSF in neurologically abnormal dogs.