Endonasal sinus surgery aims at the preservation of a lining mucosa in the reventilated and redrained cavities. It can, therefore, be confined to the removal of narrowing bone at the "isthmus" of the ducts or windows. Transnasal ethmoidectomy for diffuse polyposis consists of the removal of the ethmoidal cell septa, including the middle turbinate, and a broad fenestration of both the sphenoid sinus and the frontal infundibulum. A consequent postoperative care provided, transnasal ethmoidectomy offers excellent clinical results. A new suction-irrigation endoscope and refined instruments contribute to improved surgical exposure and to the avoidance of complications.