The renaissance of endonasal microsurgery is not only based on a deeper knowledge of the mucociliar clearance and better operation instruments but depends as well as on the improved radiological diagnosis provided by computed tomography. Since 1986, high-resolution computed tomography in two plains has been part of the authors' preoperative routine diagnostic program for paranasal sinus diseases at their clinic. One hundred and fifty-eight computed tomograms of patients with chronic rhinosinusitis, complicated acute rhinosinusitis, and polyposis nasi were reexamined. The involvement of the paranasal sinuses and abnormalities of the lateral nasal wall were analyzed. The anterior ethmoid is the most frequently affected area in all of the diseases. The computed tomogram is most important for the rhinosurgeon. The ethmoid cell system can be visualized without superimposing structures. Therefore even circumscript pathologic mucosa alterations and the extent of the disease can be defined. All this enables the surgeon to put forward an exact diagnosis and to plan a safe operation as regards important physiological structures of the nose and paranasal sinuses.