Anterior decompression of a cervical ossified posterior longitudinal ligament (OPLL) which involves the dura mater occasionally results in a postoperative cerebrospinal fluid fistula. Bone window computed tomography (CT) was carried out to detect dural defects in 21 patients with cervical OPLL. Twelve of 21 patients had double-layer OPLL, and nine had single-layer OPLL. Dural defects were present in 10 of the 12 patients with double-layer OPLL compared with only one of the nine patients with single-layer OPLL. In addition, greater stenosis of the spinal canal by the OPLL was also associated with dural defect. Preoperative bone window CT can indicate likely locations of dural defect in patients with OPLL.