목적: Understanding the anatomy of paranasal sinuses and their variations is essential to a safe and effective endoscopic sinus surgery. Ethmomaxillary sinus (EMS) is a relatively rare paranasal sinus anatomic variation. The aim of this study was to investigate the clinical significance and the imaging anatomical characteristics of the EMS. In addition, we investigated the differences between the EMS and Haller cell. 방법:We conducted a retrospective analysis of PNS CT scans that were obtained between April 2020 to December 2020 in Rhinologic Clinic at Guro Hospital. PNS CT scans (SOMATOM Definition AS Plus, Siemens, Germany) were obtained at 1 mm-section thickness. The prevalence and general characters of the EMS, co-existing anatomical variations and relationship with paranasal sinusitis were investigated. Two examiners independently analyzed the PNS CT images with the OnDemand3D software program (Cybermed, Seoul, South Korea) to measure the volume of maxillary sinuses. We compared these findings with those of Haller cells. 결과:EMS was present in 26 patients of the 250 patients (10.4%). The male to female ratio was the same. Their ages ranged between 18 and 83 years (mean age, 49.4). The EMS was bilateral in 34.6% and unilateral in 65.4%. The prevalence of Haller cells was similar to that of EMS (10.8%). Maxillary sinus hypoplasia was more frequent in co-existing anatomic variations of EMS than in Haller cells. In the analysis of unilateral EMS patients, the EMS side significantly reduced the maxillary sinus volume compared to the opposite side, whereas in the Haller cell, the difference was not significant. There were no significant relations between EMS or Haller cell and sinusitis. 결론:EMS needs to be distinguished from Haller cells. Accurate confirmation of EMS is the key to complete ventilation opening and removing of pathologic lesions in the maxillary sinus.