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A total of 1015 patients participated in three 1-week, multicenter, double-blind, randomized, placebo-controlled trials undertaken to assess the therapeutic efficacy and tolerability of twice daily administration of a nasal spray containing a combination of levocabastine (0.5 mg/mL) and oxymetazoline (0.5 mg/mL) (levocabastine-D) versus that of either agent alone in the treatment of ragweed-induced seasonal allergic rhinitis. As these studies shared a common protocol, the data have been pooled. Patient assessments revealed that the mean change in area under the curve (AUC) from baseline over the entire treatment period was significantly greater in patients treated with levocabastine or levocabastine-D than in those receiving placebo for all symptoms evaluated (nasal congestion; P < 0.05 and sneezing, rhinorrhea, nasal itching, ocular symptoms, total key symptoms, total all symptoms; P < 0.001). Corresponding changes in patients treated with oxymetazoline alone did not attain statistical significance. Day-by-day analysis demonstrated that the beneficial effects seen with levocabastine and levocabastine-D were maintained throughout the treatment period for all symptoms except nasal congestion (Days 1 and 2 only); oxymetazoline provided significant relief from nasal congestion only and only on the first day of treatment. Investigator assessments revealed similar trends. Global evaluations of therapeutic efficacy revealed that 44% of levocabastine-treated patients and 52% of patients treated with levocabastine-D considered therapeutic efficacy to be excellent or good compared with 39% of those on oxymetazoline and 26% on placebo (P < 0.01 versus placebo). Adverse experiences were reported by 30% of levocabastine-treated patients, 40% of patients treated with levocabastine-D and oxymetazoline, and 32% of placebo controls, with no statistically significant intergroup differences in incidence or type. In conclusion, twice daily levocabastine nasal spray is effective and well-tolerated for the treatment of ragweed-induced seasonal allergic rhinitis with an adverse effect profile comparable with that of placebo. Addition of oxymetazoline to the topical antihistamine does not appear to provide significant additional clinical benefit compared to that observed with levocabastine alone, and tachyphylaxis to the decongestant effect of the topical vasoconstrictor occurs within days of treatment initiation.