Seventy patients with seasonal allergic rhinitis and with positive skin test to birch pollen were treated during the birch pollen season for three weeks in a randomized, double-blind placebo controlled, multicenter study. Nineteen patients were treated with beclomethasone dipropionate aerosol (50 micrograms/puff) one puff per nostril X 4 (daily dose 400 micrograms), 20 patients with beclomethasone dipropionate aerosol two puffs per nostril X 2 (daily dose 400 micrograms), 15 patients with placebo one puff per nostril X 4, and 16 patients with placebo two puffs per nostril X 2. Efficacy of the therapy was assessed through rhinoscopy (swelling and secretion of the mucous membrane) and measuring the nasal peak expiratory flow (NPEF) before, during and at the end of the trial. Nasal symptoms and the use of antihistamine tablets were recorded for five days before the trial and daily throughout the trial. Atmospheric pollen concentrations were recorded daily and the treatment was started two days after the air pollen counts had exceeded 10/m3. The severity of the nasal symptoms was compared in the statistical analysis before the trial, during the pollen season without treatment and at the time of peak pollen counts during the treatment. Both active regimens controlled the symptoms effectively (p less than 0.01) and both placebos showed no significant effect on the symptoms. In rhinoscopy, both active drugs reduced the swelling of the mucous membrane and the difference was significant (p less than 0.05). No statistical differences were found in the NPEF or in the use of antihistamine tablets between the four groups. In conclusion this study showed that two-dose beclomethasone dipropionate with a daily dose of 400 micrograms is effective in the treatment of seasonal allergic rhinitis.