Acquired intradural arachnoid cystic lesions of the spine have been associated with trauma, hemorrhage, parasitic infections, and other insults that cause inflammation and subarachnoid adhesions. The authors describe the case of a previously healthy 36-year-old woman who presented with a chronic myelopathy due to the progressive development of a giant spinal arachnoid cyst that resulted after the intrathecal injection of phenol for the management of chronic upper extremity pain. Neurological examination, spinal computed tomography, and magnetic resonance imaging were used for diagnostic and follow-up purposes. Even after the initial excision of the cyst, the patient remained symptomatic with minimal functional recovery.