Maxillary sinus ostium may be located anywhere along the ethmoid infundibulum (middle meatus). In rhinosinusal pathology we may find an accessory ostium of the maxillary sinus due to chronic inflam­mations or after previous endoscopic sinus surgery. Existence of the accessory ostium leads to a recirculation mechanism of sinus secretions and a very difficult to treat rhinosinusal simptomatology. For every patient with this pathology we performed an endoscopic exam of the nose and a rhinosinusal CT scan. Treatment is strictly surgical by creating a unique ostium (consisting of both primary and accessory ostium). Endoscopic anatomy of the ostiomeatal unit should be well understood by the surgeon in order to perform a correct endoscopic sinus surgery, obtaining a proper sinus ventilation and avoiding complications. The presence of an accessory maxillary sinus ostium has only an endoscopic surgical treatment for ensuring sinus drainage and increasing the quality of life of our patients. Keywords: