A five-year-old domestic longhair was presented with hind-limb ataxia and some degree of incontinence of two weeks' duration. An enlarged spinal canal from the twelfth thoracic (T12) vertebra to the third lumbar (L3) vertebra was identified on survey radiographs. An intradural-extramedullary cavity at the twelfth (T12) and thirteenth (T13) thoracic vertebrae, filled with contrast material, was demonstrated on myelography. A left-sided hemilaminectomy was performed over this region, and a subarachnoid cavitation or cyst was found to be the cause of the severe spinal-cord compression. The cyst was drained. The cat showed improvement in the neurological signs during the first three weeks postoperatively. Six months later, no neurological deficits were identified on follow-up examination.