Spinal interdural cyst is an extremely rare lesion, the wall of which consists of a dura-like layer without arachnoid. This report describes two cases in which patients with interdural cysts of the thoracolumbar spine presented with atypical neurologic signs and symptoms, including lower back pain and radiating pian in both lower extremities. Plain radiographs, CT myelograms and magnetic resonance images were obtained, but the cysts could not be differentiated from extradural arachnoid cyst. Surgical intervention revealed a dura-like layer of cyst wall, and within the cyst, a fluid resembling cerebrospinal fluid was present. In one case, microscopic examination showed that fragments of connective tissue without an arachnoid lining formed a thin fibrous cystic wall, a finding consistent with meningeal cyst.