BACKGROUND Azelastine nasal spray and oral cetirizine are selective histamine H(1)-receptor antagonists that are approved in the United States for the treatment of seasonal allergic rhinitis (SAR). OBJECTIVE The objective of the present study was to compare the efficacy and tolerability of azelastine nasal spray administered at the recommended dosage of 2 sprays per nostril twice daily with those of cetirizine in the treatment of moderate to severe SAR. METHODS This multicenter, randomized, double-blind, parallel-group, 2-week comparative study was conducted during the 2004 fall allergy season in patients with moderate to severe SAR. After a 1-week placebo lead-in period, patients were randomized to receive azelastine nasal spray 2 sprays per nostril twice daily plus placebo tablets or cetirizine 10-mg tablets once daily plus a placebo saline nasal spray for the 2-week double-blind treatment period. The primary efficacy variables were (1) change from baseline to day 14 in the 12-hour reflective total nasal symptom score (TNSS), which combines scores for rhinorrhea, sneezing, itchy nose, and nasal congestion, and (2) onset of action, based on the instantaneous TNSS over 4 hours after the first dose of study drug. During the double-blind treatment period, patients recorded their symptom scores on diary cards twice daily (morning and evening). Patients aged > or =18 years also completed the Rhinoconjunctivitis Quality of Life Questionnaire (RQLQ) at baseline and on day 14. RESULTS Three hundred seven patients were randomized to treatment, and 299 completed 2 weeks of study treatment. The age of the population ranged from 12 to 74 years (mean, 35 years), 62.9% were female, and 69.6% were white. Over 2 weeks of treatment, both groups had significant improvements in the TNSS compared with baseline (P < 0.001). The overall change in TNSS was significantly greater with azelastine nasal spray compared with cetirizine (29.3% vs 23.0% improvement, respectively; P = 0.015). In terms of onset of action, azelastine nasal spray significantly improved the instantaneous TNSS compared with cetirizine at 60 and 240 minutes after the initial dose (both, P = 0.040). Scores on each domain of the RQLQ were significantly improved in both groups compared with baseline (P < 0.001); the overall RQLQ score was significantly improved with azelastine nasal spray compared with cetirizine (P = 0.049). Both treatments were well tolerated. CONCLUSIONS In this 2-week study in patients with moderate to severe SAR, azelastine nasal spray was well tolerated and produced significantly greater improvements in TNSS and total RQLQ score compared with cetirizine.