Bronchial epithelial damage occurs regularly in bronchial asthma, but it is not known whether such damage occurs in the mucosa of nasal polyps. We obtained nasal polyp tissues from thirty patients and we examined these tissues for evidence of epithelial damage. Immediately after resection, polyp tissue was fixed in Karnovsky's fixative, embedded in methacrylate and stained with Giemsa pH 6-5. Normal nasal tissue from eight patients undergoing nasal septal reconstruction was similarly processed. As a disease control, we examined tissue from eight patients with nasal polyps associated with cystic fibrosis. Tissues were viewed by microscopy and epithelial damage was expressed as the percent of surface involved. Twenty-eight of the thirty patients with idiopathic nasal polps (93%) showed complete loss of nasal mucosa in varying degrees (range 3-81% of surface; mean, 29%). All patients showed evidence of some epithelial damage, either complete loss or marked desquamation (range 9-99% of surface; mean 54%). In contrast, six of eight biopsies from patients undergoing septal reconstruction and five of eight nasal polyps from patients with cystic fibrosis showed little or no evidence of epithelial damage. The results indicate that nasal polyps regularly show evidence of epithelial damage similar to that seen in bronchial asthma, and this abnormality may partly explain the rhinorrhea which is prominently associated with nasal polyps.