Ciclesonide is an intranasal corticosteroid approved for the treatment of allergic rhinitis. We conducted a randomized, double-blind, parallel-group, placebo-controlled study to evaluate the time to onset of action of ciclesonide 200 μg once daily in 502 adults with seasonal allergic rhinitis of at least 2 years' duration. To trigger immunologic priming, patients underwent between one and five priming sessions with exposure to 3,500 grains/m3 (±500) of ragweed pollen in an environmental exposure chamber. The criteria for a successful priming session were a patient-assessed instantaneous total nasal symptom score of at least 6 (of a possible 12) and a nasal congestion or rhinorrhea score of at least 2 (of a possible 3) 90 minutes after allergen exposure during at least two consecutive priming sessions. Patients were then randomly assigned to receive either a single dose of ciclesonide 200 μg (n = 251) or placebo (n = 251) administered intranasally. The difference in the change from baseline total nasal symptom scores in the two groups was assessed hourly for 12 hours after administration. Onset of action was determined to have taken place the first time that the effects of ciclesonide, as reflected in the total nasal symptom score, were significantly greater than those of placebo at a particular hourly assessment, provided that the subsequent hourly assessment also showed a statistically significant difference. The onset of action of ciclesonide occurred within 1 hour of administration (p = 0.01 vs. placebo), and the significant difference in total nasal symptom scores between ciclesonide and placebo was maintained through post-treatment hour 12 (p = 0.018).