The ostiomeatal complex is responsible for the clearance of most sinus secretions. To evaluate the delayed effects of irradiation, this study examined the infundibulum mucosa of 10 patients who developed sinusitis after receiving radiotherapy for nasopharyngeal carcinoma (NPC). Pathologic findings under the light microscope revealed an increased deposition of dense collagenous fibers in the lamina propria. The epithelial cells also transformed into a stratified arrangement and showed gradual reduction of cytoplasmic volume. Ultrastructural observations detected areas of ciliary loss, intercellular and intracellular vacuolation, and ciliary dysmorphism. Most of these pathologic findings were observed even in a patient 23 years after irradiation. The results presented herein suggest that radiotherapy may cause long-term damage to the nasal epithelium that may be responsible for the prolonged sinusitis of irradiated NPC patients.