Objective: To evaluate the efficacy and tolerability of desloratadine 5mg once daily, a new, selective, H-1-receptor antagonist, for the treatment of patients with seasonal allergic rhinitis (SAR) during the two major pollen seasons in the USA. Design: Two multicentre, randomised, double-blind, placebo-controlled, parallel-group investigations in patients with SAR are reported, one conducted during the spring (172 and 174 patients in the desloratadine and placebo groups, respectively) and the other during the fall (164 patients each in the desloratadine and placebo groups) allergy season. Study Participants: Patients 12 years of age or older with clinically symptomatic SAR and a minimum 2-year history of SAR. Interventions: Desloratadine 5mg or placebo once daily for 14 days following a 1-week screening period. Main Outcome Measures: The primary efficacy assessment was the mean change from baseline in the average reflective am/pm total symptom score (TSS) averaged over the 2-week study period. Results: In both seasons, desloratadine 5mg once daily resulted in a significant improvement in TSS for patients with SAR (p < 0.01 and p = 0.02, respectively) over the a-week study. Adverse events reported were mild to moderate in severity and similar to placebo. Assessment of sedation and ECG data revealed no clinically significant changes from baseline with desloratadine- or placebo-treated patients. Conclusion: Desloratadine 5mg once daily was effective and well tolerated in the treatment of symptoms associated with SAR following the first dose of therapy and continuing for the 2-week duration of the study during both the spring and fall allergy seasons.