This study examined the seasonal effects on eosinophils and secretory responsiveness of the nasal mucosa in 22 patients with allergic rhinitis due to birch pollen (11 patients received placebo and 11 budesonide, 200 micrograms once daily in each nostril). The pollen counts during the study season were too low to produce a significant symptomatology. Hence, our findings demonstrate threshold alterations of the airway mucosa in allergic rhinitis and their inhibition by anti-inflammatory drug intervention. The patients were monitored for 8 weeks with daily recordings of pollen counts and symptom scores. Once every week a series of laboratory tests was carried out: the local eosinophil influx was determined using a Rhinobrush technique; the levels of eosinophil cationic protein (ECP) were analysed in nasal lavage fluids; and the secretory response to intranasal methacholine was measured. Treatments started after a 2-week run-in period. The proportion of eosinophils increased markedly in the placebo group and was elevated also during the last two study weeks when the pollen counts were practically nil. The secretory responsiveness to methacholine increased during the pollen season and returned to baseline towards the end of the study period. The topical glucocorticoid treatment reduced the proportion of eosinophils, the ECP levels, and the secretory response to methacholine compared to placebo. We conclude that the increased traffic and activity of eosinophils and less conspicuously the increased secretory responsiveness are expressions of the mucosal inflammation that precede the development of symptoms in seasonal allergic rhinitis.