Oily contrast medium had been in use since the early 19th century as a radiographic agent for detecting spinal lesions and spinal cord tumors until the late 20th century. At that point computed tomography scanning and magnetic resonance imaging, or other hydrophilic contrast medium substituted for it. Adverse effects of oil-based dye, both acute and chronic, had been reported since the middle of the 20th century. In this paper the authors report the case of syringomyelia that seemed to be caused mainly by remaining oily contrast medium for 44 years. Syringomyelia secondary to adhesive arachnoiditis caused by oily contrast medium after a long period of time is well known. In the present case, however, surgery revealed only mild arachnoiditis at the level of syringomyelia as well as both solid and liquid remnants of contrast medium. Generally, cerebrospinal fluid (CSF) blockage due to an arachnoid adhesion is considered to cause syringomyelia following adhesive arachnoiditis. The authors speculated that in the present case syringomyelia was induced by a mechanism different from that in the previously reported cases; the oily contrast medium itself seems to have induced the functional block of CSF and impaired the buffer system of the intrathecal pressure. No reports on thoracic adhesive arachnoiditis and syringomyelia caused by oil-based dye referred to this mechanism in reviewing the literature.