Material and Methods: A case of 60-year-old woman with idiopathic syringomyelia and progressive myelopathy by dorsal arachnoid web is discussed. One focal indentation along the dorsal surface of the spinal cord at the T3 vertebral body level revealed the scalpel sign, a radiologic entity diagnostic of a dorsal thoracic arachnoid web. Results: T2-T3 laminectomy for the resection of dorsal arachnoid band was performed. Careful sectioning of the web resulted in visual apparent relief of the compression and good functional recovery. Conclusion: In patients with presumed idiopathic syringomyelia, imaging studies should be closely inspected for the presence of transverse arachnoid web. This extramedullary transverse band of arachnoid tissue extends to dorsal surface of the spinal cord, resulting in mass effect and dorsal indentation, known as scalpel sign because of its apparent resemblance to a scalpel on sagittal imaging. Early diagnosis with early intervention may benefit greatly patients. Surgical resection of transverse arachnoid web is a minimally invasive procedure with low morbidity, and can result in resolution of syringomyelia and improvement in neurological function.