Generation of clusters of free eosinophil granules (Cfegs), through lysis of eosinophils, has recently been proposed as a major paradigm for ultimate activation of airway mucosal eosinophils. In the present study involving patients with seasonal allergic rhinitis, we have investigated whether generation of Cfegs in the nasal mucosa is a feature of allergic rhinitis. Nasal mucosal biopsies were obtained before and late in a birch-pollen season, and were subjected to histochemical staining of eosinophil peroxidase. In biopsies obtained before the pollen season, a few, intact eosinophils were observed, and Cfegs were scarce. In biopsies taken during the pollen season, the numbers of eosinophils were increased about 10-fold (P < 0.05) and the Cfegs about 25-fold (P < 0.05). We conclude that generation of Cfegs is a significant feature of seasonal allergic rhinitis and that this process represents the ultimate activation of mucosal eosinophils in this disease.