This multicenter, double-blind, placebo-controlled trial evaluated the efficacy and safety of levocabastine nasal spray, a potent and selective H1-receptor antagonist, in the control of histamine-mediated symptoms of seasonal allergic rhinitis. Adults with > or = 2 year history of allergic rhinitis due to Mountain Cedar were randomized to treatment with levocabastine nasal spray (0.2 mg twice daily) or placebo for 28 days during the 1994-1995 Mountain Cedar allergy season. Patients assessed the severity of their rhinitis symptoms on a four-point scale twice daily. At the end of the trial, patients also performed a global evaluation of treatment efficacy on a five-point scale. Overall for the 4-week treatment period, levocabastine nasal spray significantly reduced major nasal (runny nose and sneezing) and primary rhinitis (runny nose, sneezing, and itchy/gritty eyes) symptoms compared with placebo on both repeated measures (p = 0.023; p = 0.01) and ANOVA (p = 0.003; p < 0.001) analyses. Global evaluations of treatment efficacy at the end of the trial significantly favored levocabastine over placebo (p = 0.002). Overall, the incidence of adverse events was similar for both treatment groups. In general, most adverse events were mild in intensity, with sinusitis (17% each group), headache (17% placebo, 14% levocabastine), and rhinitis (8% placebo, 2% levocabastine) most commonly reported. Levocabastine nasal spray 0.2 mg twice daily was significantly more effective than placebo in the relief of histamine-mediated symptoms in patients with seasonal allergic rhinitis and was well tolerated over the 28-day treatment period.