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Sponsorships or competing interests that may be relevant to content are dis-closed at the end of this article. Objective. To evaluate the prevalence of the persistent pet-rosquamosal venous sinus in patients with chronic otitis media using a high-resolution CT scan. Study Design. Retrospective chart review. Setting. Tertiary referral center. Subjects and Methods. Eighty-four patients older than 6 years of age underwent surgery (in a total of 92 ears) for chronic otitis media at Kyoto University Hospital, Department of Otolaryngology-Head and Neck Surgery. We used high-resolution CT scans to evaluate the prevalence and size of persistent petrosquamosal sinuses, as well as their relative position to the middle fossa. Results. A petrosquamosal sinus was detected in 42 (45.7%), a higher frequency than in ears without chronic otitis media (10.3%). The diameter of the sinuses was \1.0 mm in 20 ears, 1.0 mm to 2.0 mm in 17 ears, and.2.0 mm in 5 ears. The petrosquamosal sinus was positioned inferior to the lowest part of the middle fossa in 10 ears. Conclusion. The petrosquamosal sinus was detected fre-quently in high-resolution CT scans in patients with chronic otitis media. In approximately half of the patients, the sinus was larger than 1 mm in diameter and may be encountered during mastoidectomy. Keywords petrosquamosal sinus, chronic otitis media, high-resolution CT sca