Background: Topical nasal corticosteroids have become a mainstay of treatment for the symptoms of seasonal allergic rhinitis (SAR). It is likely that topical corticosteroids, by blocking an initial influx of inflammatory cells in the nasal mucosa induced by aeroallergens, may have a preventive effect on nasal allergy symptoms when administered before the pollen season. Objective: This study was designed to assess the efficacy and safety of an 8-week course of mometasone furoate nasal spray (MFNS), 200 μg once daily, in the treatment of SAR compared with beclomethasone dipropionate aqueous nasal spray (BDP), 168 μg twice daily, and placebo vehicle, when treatment is initiated before the anticipated onset of the ragweed season. Methods: Three hundred forty-nine patients with SAR to ragweed pollen from nine centers in the Northeast and Midwest of the United States were randomized to one of the three intranasal study medications (MFNS, 200 μg once daily, BDP, 168 μg twice daily, or placebo vehicle), starting 4 weeks before the estimated start of the ragweed season. Results: The proportion of “minimal symptom” days (total nasal symptom score ≤2) was statistically significantly higher in both the MFNS and BDP groups when compared with the placebo vehicle group (p < 0.01). The two active treatment groups were not statistically significantly different from each other. MFNS and BDP displayed a similar safety profile that did not differ from placebo. Conclusions: This suggests that MFNS, 200 μg (once daily), is a useful therapy in the prophylactic treatment of SAR. (J ALLERGY CLIN IMMUNOL 1996;98:724-31.)