The main purpose of endoscopic sinus surgery is the improvement of ventilation & drainage of sinuses by opening the obstructed ostium of sinuses. But it is not well known that how much the opened natural ostium contribute to the normalization of sinus mucosa. We could evaluate preoperative & postoperative OMU CT of the 38 maxillary sinuses(27 patients) which have well opened natural ostium after endoscopic sinus surgery. The degree of improvement in sinus mucosa based on preop. & postop. OMU CT findings and the possible related factors which might affect the improvement of sinus mucosa were studied objectively. The state of maxillary sinus was divided into 4 grades according to the portion of the area of diseased mucosa divided by the total area of maxillary sinus. The overall improvement rate of mucosa of maxillary sinus was 64.5%. In case of preop. grade IV, 35.7% still showed grade III after endoscopic sinus surgery. In case of preop. grade III, 71.4% showed grade II. In case of preop. grade II, 33.3% showed grade I. We concluded that the worse preop. mucosal state, the poor the degree of improvement. Allergy, interval between preop. & postop. OMU CT, duration of medication and preop. extent of polyposis showed no statistical relationships with the mucosal improvement of the maxillary sinus.