OBJECTIVE To establish the prevalence and individual variations of the ethmomaxillary sinus (EMS) using cone beam computed tomography (CBCT). METHODS CBCT images from 553 dental and ENT patients (1106 sides), presenting to treatment at Minsk outpatient clinics, Belarus, were used in this study. The maximum vertical diameter of ethmomaxillary sinus in the coronal plane was measured. Anatomical variations of the ostiomeatal complex and mucosal diseases of paranasal sinuses were recorded. RESULTS Ethmomaxillary sinus - is posterior ethmoidal cell extending to the posterior superior part of the maxillary sinus (MS) while draining into superior meatus. It was present in 13 of 553 patients (2.4%). The age of patients with EMS ranged from 12 to 60 years, including 8 males and 5 females. A total of 5 patients had unilateral EMS and 8 patients - bilateral EMS. Ethmomaxillary sinus was extended to the alveolar bone in five cases. Moreover, in two cases, the roots of the upper third molars protruded into its lumen. CONCLUSIONS The enlarged posterior ethmoidal air cell can occupy the posterior superior portion of the maxillary sinus and even reach the alveolar bone. Such a cell is called the ethmomaxillary sinus and, as a rule, its presence is accompanied by excessive pneumatization of other cranial bones. Inflammation of the mucous membrane of the ethmomaxillary sinus is most often not combined with the radiographic signs of maxillary sinusitis, and a direct communication of the EMS and MS was detected in only one patient.