The case of a pacient with thoracic arachnoideal cyst is reported. There was a neurological picture of a spinal cord compression in thoracic level with analgesia and anesthesia in T12 and L1 at left. Radiographs showed compression of the pedicles with widening of interpedicular spaces of the eight to twelfth thoracic vertebrae. The myelogram and the laminectomy confirmed the hypothesis of aracnoideal cyst. This case of extradural cyst of the spinal canal is added to the 76 previously described in the literature. The incidence, etiology, phisiopathology, clinical-picture, diagnosis, surgical treatment and prognosis are discussed.