Paraplegia after a routine lumbar laminectomy: report of a rare complication and successful management. Arachnoid cysts of the spinal canal are relatively common lesions that may be either intra- or extradural. These cysts are usually asymptomatic but may produce symptoms by compressing the spinal cord or nerve roots. We report a case in which an intradural thoracic arachnoid cyst became symptomatic after a routine decompressive lumbar laminectomy for spinal stenosis. Myelography revealed no abnormality, although magnetic resonance imaging and computed tomography after myelography demonstrated a mass within the posterior aspect of the thoracic spinal canal associated with anterior displacement and compression of the spinal cord. A change in the flow dynamics of the cerebrospinal fluid probably allowed the development of spinal cord compression due to one of the following: expansion of the cyst, decreased cerebrospinal fluid buffer between the cord and the cyst, or epidural venous engorgement. A concomitant and more cephalad lesion such as an arachnoid cyst should be considered when myelopathic complications arise after lumbar surgery. Magnetic resonance imaging and computed tomography after myelography are useful to demonstrate the additional pathological processes. 