The purpose of this study was to compare the preoperative and postoperative ultrastructural changes of paranasal sinus mucosa in patients treated with functional endoscopic sinus surgery for chronic rhinosinusitis. Twelve patients undergoing functional endoscopic sinus surgery for the treatment of chronic rhinosinusitis were involved. The ethmoid sinus mucosa was sampled during the operation and similar to6 months after the operation. The ciliated epithelium of sinus mucosa was evaluated with transmission electron microscopy. The samples were taken at the Otolaryngology Department of Istanbul University School of Medicine. Electron microscopic study was performed at the Histology and Embryology Department of the same University. Preoperatively, ciliated epithelial cells of the Sinus mucosa of the patients showed deqenerated ultrastructure with decreased number of cilia, cytoplasmic protrusions, cisternal dilatations of endoplasmic reticulum, and mitochondrial swellings. Remnants of degenerated cells and cellular separations at cell junctions were evident in the diseased epithelium. Goblet cells were frequent along the epithelial lining. Postoperatively, normal architecture and ultrastructure of the ciliated epithelium was restored. These observations showed that unlike other surgical operations, paranasal sinus mucosa can regenerate and the ciliated epithelium can return to normal after functional endoscopic sinus surgery.