The authors describe two occurrences of idiopathic spinal cord herniation, an entity that has been reported previously in only 11 cases. The patients described in this report presented in midlife with Brown-Séquard syndrome. Computerized tomography myelography and magnetic resonance (MR) imaging showed ventral displacement of the spinal cord with no apparent dorsal mass. Intraspinal cerebrospinal fluid (CSF) flow studies in which phase-contrast pulse sequence cine MR imaging was used displayed a normal pattern dorsal to the spinal cord. Percutaneous intrathecal endoscopic observation did not disclose dorsal intradural cysts, but ventral adhesions were seen between the spinal cord and the dura. Microsurgical intradural exploration revealed ventrolateral herniation of the cord and a ventral root through a dural defect into an epidural arachnoid cyst. The adhesions were released, the cord was repositioned intradurally, and the dural defect was patched. The patients showed gradual improvement postsurgery, as did most of the patients in the previously reported cases. The CSF flow and endoscopic studies were found to be particularly informative, and dural patching with surgical membrane is recommended.