The mechanisms by which eosinophilic inflammation damages the epithelium and contributes to recurrent acute exacerbations in chronic rhinosinusitis (CRS) have not been fully elucidated. We tested the hypotheses that eosinophils deposit toxic major basic protein (MBP) in the mucus and that MBP reaches concentrations able to damage the sinonasal epithelium. Tissue specimens with mucus attached to the tissue were carefully collected from 22 patients with CRS and examined by using immunofluorescence staining for MBP. This immunofluorescence was digitally analyzed to determine the area covered by MBP and the intensity of the staining (estimating MBP concentration). Levels of MBP in extracts from nasal mucus were quantitated by means of RIA. Heterogeneous eosinophilia was evident within tissue and mucus specimens. All tissue specimens showed intact eosinophils, but diffuse extracellular MBP deposition, as a marker of eosinophil degranulation, was rare. In contrast, all mucus specimens showed diffuse MBP throughout and abundant diffuse extracellular MBP deposition within clusters of eosinophils. Digitized analyses of MBP immunofluorescence revealed increased area coverage ( P < .0001) in mucus compared with that seen in tissue. Estimated concentrations of MBP within the clusters suggested toxic levels. MBP concentrations in mucus extract reached 11.7 μg/mL; MBP was not detectable in healthy control subjects. In patients with CRS, eosinophils form clusters in the mucus where they release MBP, which is diffusely deposited on the epithelium, a process not observed in the tissue. Estimated MBP levels far exceed those needed to damage epithelium from the luminal side and could predispose patients with CRS to secondary bacterial infections.