Functional endoscopic sinus surgery has been helpful to manage chronic or recurrent paranasal sinusitis, but extended ethmoidectomy may prevent the natural passage of secretion and mucociliary tranport from frontal, maxillary and even sphenoid sinus. Ethmoidectomized cavities have many problems in healing process. The purpose of this study was to evaluate the regeneration of epithelium in ethmoidectomized cavity after functional endoscopic sinus surgery and to investigate the problems of healing process. Fifty six specimens from ethmoidectomized cavities were taken from postoperative 3rd week to 24th week and then were observed by light and electron microscopy. One hundred ethmoidectomized cavities undergoing healing process were observed with an endoscope. On endoscopic examination, granulation tissue, edema and polyps were observed at the postoperative 3rd and 6th week, and adhesion and progressive stenosis were common after postoperative 12th week. However, complete epithelial regeneration was observed from postoperative 12th week to 24th week on histopathologic examination. Therefore, careful inspection until postoperative 6th month is essential to manage problems occurring after endoscopic sinus surgery with extended ethmoidectomy.