Treatment with beclomethasone dipropionate aerosol (BDA), 50 mug four times daily in each nostril, was compared with placebo therapy in a double-blind non-crossover trial of 30 matched patients with allergic rhinitis induced by ragweed pollen. The trial was started at the beginning of the ragweed season and continued for 42 days. Response to treatment was assessed from information on daily diary cards, weekly objective measurements of nasal patency and measurement of total eosinophil count (TEC) before treatment and at week 4. Patients in the BDA group had significantly less (P less than 0.05) sneezing, rhinorrhea and nasal stuffiness at 36 days, cough at 10 days and antihistamine consumption at 17 days. There was no significant difference between the groups in eye symptoms, nasal airway inspiratory resistance, maximum inspiratory nasal flow or TEC. Overall comparison with previous pollen seasons by the patients indicated moderate to great improvement in 86% of the BDA group and in 13% of the placebo group (P less than 0.01). Minor side effects were noted by two patients in each group.