Astrocytomas comprise the majority of canine gliomas, but are uncommon in cats. It occurs most commonly in the pyriform lobe, convexity of the cerebral hemisphere, thalamus-hypothalamus, and midbrain and occasionally in the cerebellum and spinal cord. A six year-old male Siamese cat was presented to a veterinary clinic with hemiparesis, atrophy and muscle pain in right thoracic limb. The neurologic exam showed increased reflexes, hyperesthesia, proprioceptive deficit and spastic paresis. Plain radiographic was realized and no alteration was observed. The patient was submitted to steroidal and analgesic therapy. After four days clinical signs had progressed to pelvic limbs with increased pain; therefore the owner opted for the euthanasia. Necropsy was performed and selected tissues were fixed in 10% buffered formalin solution and routinely processed for histological and immunohistochemical evaluation. Immunohistochemical assay to glial fibrillary acidic protein (GFAP), vimentin, S100 protein, lysozyme and macrophages (LN-5) was also performed. At necropsy, focal softening at C6 segment of the spinal cord was observed. No other gross lesions were observed. Microscopic examination of the neoplasm revealed diffuse proliferation of large cells with abundant eosinophil cytoplasm and eccentrically located large nuclei with prominent nucleoli. Atypia was moderate and the mitotic index was low. The neoplastic cells were highly immunoreactive for GFAP, vimentin, and S100 protein, with no immunoreactivity for LN-5 and lysozyme. The findings were consistent with a diagnosis of gemistocytic astrocytoma. The prognosis of animals with tumors of the central nervous system is reserved, but it depends on tumor location, surgical accessibility, growth and degree of damage to the nervous damage. The prognosis for cats with intramedullary spinal astrocytoma is poor and the disease has a relentlessly progressive course. The clinical signs, laboratorial and radiographic exams generally are not conclusive. The histopathological and immunohistochemical methods are fundamental to the definitive diagnosis of intramedullary tumor of spinal cord.