Fourteen unselected adult patients with nasal polyps had ultrastructural examination of mast cells from matching biopsies of the polyp and inferior turbinate. Between three and 10 blocks were examined for each patient in both tissues and every mast cell that had a nucleus was photographed for study. Fifty-three mast cells were found within the stroma of nasal polyps and 54 in the submucosa of the inferior turbinate biopsies. The number of granules ranged between 13 and 167 (mean 60) for polyps and 18 and 148 (mean 61) in the inferior turbinate. The mast cells appeared essentially normal in the inferior turbinate of four patients. The degree of degranulation of the mast cells was calculated as in previous studies and then averaged for both the polyp and the inferior turbinate of each patient. There was greater degranulation in the nasal polyp compared to inferior turbinate (p= 0.03). These results were compared with mast cell degranulation found in the normal nose and in the inferior turbinate of patients with perennial allergic rhinitis which we previously published. The inferior turbinates in these patients were more degranulated than the normal nose (p= 0.0001) but were similar to that found in patients with perennial allergic rhinitis. This suggested that some degree of degranulation may occur throughout the nose in two thirds of the patients with nasal polyps which supports the theory that mast cell reactions are not limited to the polyps in a proportion of patients.