Mometasone furoate nasal spray (MFNS, NASONEX ), is a new synthetic corticosteroid with considerable efficacy in the treatment of seasonal and perennial rhinitis and less than 0.1% systemic absorption. This study was designed to evaluate the time of onset of action of MFNS. The subjects were evaluated over the course of 2 weeks during the spring allergy season. The effects of MFNS 200 microg given once daily for 2 weeks were evaluated in a randomized, multicenter, double-blind, placebo-controlled study in 201 patients with seasonal allergic rhinitis. Clinically significant onset of action was assessed prospectively by special patient diary cards kept during the first 3 days of treatment. By 12 h after initial dosage (the earliest evaluation), 28% of patients in the MFNS group experienced clinically significant relief, compared with 13% of those given placebo (P = 0.01). Median time to at least moderate symptom relief in patients who received MFNS was 35.9 h, compared with more than 72 h in patients given placebo (P<0.01). By 72 h, 64% of the patients receiving MFNS experienced at least moderate relief, compared with 40% of those treated with placebo (P<0.01). Both patient and physician ratings of symptom severity, response to treatment, and overall condition of rhinitis indicated significant (P<0.01) superiority of MFNS over placebo. MFNS was well tolerated, with adverse events comparable to placebo. MFNS provided rapid onset of clinically significant symptom relief in patients with seasonal allergic rhinitis.