In this report, a 55-year-old Caucasian women with an arachnoid cyst of the thoracic spine is presented. This cyst remained undiagnosed because of the nonspecific nature of her symptoms over approximately three months. Only when she started to complain of ataxia, a posterior fluid collection compressing the spinal cord was found in MRI. Even though preoperative diagnosis remained uncertain, this additional neurological dysfunction warranted surgical treatment. Surgery was successful with respect to in-toto removal of the intradural, extramedullary cyst, reversal of cord compression and symptoms. Histological diagnosis was of an arachnoid cyst.