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Objective: To detect, quantify, and compare respiratory epithelial cell proliferation in nasal mucosa and polyps from patients with nasal polyposis. Design: Cohort study. Setting: Patients and samples were selected at the Hopital Intercommunal de Creteil (France). Flow cytofluorometry and immunohistochemistry were performed at Hopitaux Tenon and Mondor (Universite Paris [France] VI et XII). Patients: Twenty-one patients undergoing endoscopic ethmoidectomy for treatment of nasal polyposis. Methods: In 10 cases, epithelial cells were removed from frozen inferior turbinate mucosa and polyps by mechanical disaggregation and were then analyzed by flow cytofluorometry, providing the cell DNA content (propidium iodide labeling) and the percentage of S-phase cells. In 11 cases, inferior turbinate mucosa and polyps were fixed in formaldehyde and embedded in paraffin. Proliferating cell nuclear antigen expression in the epithelium was quantified by immunohistochemistry: a proliferating cell nuclear antigen index was calculated for each sample in the basal area, suprabasal area, and full height of the epithelium. Results: All cell populations studied were diploid, and percentages of S-phase cells were significantly higher in nasal polyps than in mucosa. Proliferating cell nuclear antigen indexes were significantly higher in nasal polyps than in the suprabasal area and full height of the mucosal epithelium. Conclusion: Cell proliferation is increased in epithelium from nasal polyps. Epithelial damage caused by inflammatory mediators could induce this increased cell proliferation via epithelial repair processes. Inflammatory cells could up-regulate epithelial cell proliferation by secreting growth factors. (Arch Otolaryngol Head Neck Surg. 1996;122:432-436)