Beclomethasone dipropionate nasal spray is widely used in the treatment of seasonal allergic rhinitis; however, the time of onset of action has not been determined. This study assessed the onset of action, level of relief, and efficacy of beclomethasone nasal spray in patients with seasonal allergic rhinitis. In a double-blind, randomized, placebo-controlled, parallel-group, multicenter, 7-day study, symptomatic patients were administered two inhalations of beclomethasone dipropionate (n = 80) or placebo (n = 81) into each nostril twice daily. Patients assessed the onset of action and level of relief at 6, 24, and 48 hours and at days 3 and 7. Investigators evaluated symptoms at days 0, 3, and 7 and response to therapy at days 3 and 7. The difference in the cumulative number of patients reporting relief of symptoms was statistically significant in favor of beclomethasone dipropionate by hour 24 (P = 0.05). Patients in the beclomethasone dipropionate group experienced a greater level of relief than patients receiving placebo at hour 24, and improvement increased over the 7-day study compared with a decrease in relief in the placebo group. Beclomethasone dipropionate was significantly more effective than placebo in reducing symptoms (P < or = 0.02), and patients in the beclomethasone dipropionate group showed a more favorable response to treatment than did patients in the placebo group (P < 0.01). Adverse events were minor in both groups. Beclomethasone dipropionate nasal spray produced significant onset of relief of symptoms the first day of treatment; improvement was sustained and increased over the course of the study.