Background: Mometasone furoate is a potent glucocorticoid that can markedly inhibit proinflammatory Th-2 cytokines in vitro. An aqueous nasal spray formulation has been shown to be clinically active in reducing the symptoms of perennial and seasonal allergic rhinitis. Objective: To determine whether pretreatment with mometasone furoate 200 E mu g once daily decreases specific indices of early and late phase nasal inflammation compared with placebo. Methods: A randomized, double-blind, placebo-controlled crossover study was conducted using nasal provocation with ragweed antigen in 21 patients with ragweed-induced allergic rhinitis out of the ragweed season; the treatment period was 2 weeks. Symptom scores, rhinoprobe cytology, and nasal lavage fluid were collected during early and late phase periods for nasal cytokines (interleukin, 1, 4, 5, 6, and 8) and leukotriene B-4 determinations using ELISA and RIA. Results: Mean nasal symptom scores and sneezing frequency were consistently lower with mometasone furoate compared with placebo. Treatment was associated with a statistically significant early phase (30-minute time point) reduction in nasal lavage histamine levels compared with placebo (14.3 versus 20.2 ng/mL, P = .02). Within-treatment comparisons suggested that mometasone furoate reduced the antigen-induced late-phase response for IL-6, IL-8, and eosinophils compared with pretreatment. There were similar, but smaller, changes seen in the placebo group for these measurements. There were no statistically significant changes following antigen challenge in IL-1, IL-4, IL-5, LTB4, or in other nasal cytology parameters. Conclusion: These results suggest that the clinical activity of mometasone furoate nasal spray in seasonal allergic rhinitis is likely due, in part, to a reduction in the levels of histamine in nasal secretions related to the early phase response, and reductions in IL-6, IL-8, and eosinophils during the late phase response.