Background. Functional endoscopic sinus surgery may be indicated when certain anatomic variations impede the normal drainage of the paranasal sinuses through the ostiomeatal complex. We aimed at studying the drainage system of the maxillary sinus which consists of the maxillary infundibulum, the main ostium of the maxillary sinus, the ethmoidal infundibulum and the hiatus semilunaris inferior.Material and methods. The study was performed retrospectively on cone beam computed tomography (CBCT) scans of 60 subjects (N=120 maxillary sinuses). The anatomical pattern of the maxillary sinus drainage was studied on coronal scans.Results. As related to different morphological possibilities in the supero-lateral limit of the maxillary sinus drainage system, five different patterns were defined: in type I (55%) there was no pneumatization in that situs, in type II (18%) there was an infraorbital recess of the maxillary sinus placed above the sinus ostium, in type III (14%) an ethmoidal recess of the maxillary sinus was expanded within the ethmoid bone, above the ethmoidal infundibulum, in type IV (3%) there were Haller cells above the sinus ostium, while in type V (10%) there were non-infraorbital ethmoid air cells above and draining into the ethmoidal infundibulum.Conclusion. It appears that CBCT is a reliable tool to make an anatomical distinction of the variable pattern of pneumatization impeding a normal drainage of the maxillary sinus, between maxillary sinus- and ethmoid-derived air-filled spaces.