To compare montelukast sodium and pseudoephedrine hydrochloride in the treatment of seasonal allergic rhinitis. Design: A 2-week, parallel, randomized, double-blind study with rolling enrollment. Setting: Tertiary care medical center. Patients: A total of 58 adult subjects with ragweed allergic rhinitis as documented by positive findings on a skin test to ragweed and history of symptoms during previous seasons. Interventions: After recording their own baseline nasal symptoms, nasal peak inspiratory flow (NPIF), and diurnal and nocturnal rhinoconjunctivitis quality of life (QOL) scores, subjects were randomized to receive daily morning oral doses of either pseudoephedrine hydrochloride (240 mg) or montelukast sodium (10 mg) for 2 weeks. They recorded their nasal symptoms and NPIF twice daily during this time, and at the end of the study, they completed another QOL questionnaire and 2 tolerability profiles. Main Outcome Measures: Nasal symptoms, NPIF, QOL scores, and tolerability profiles. Results: Both active treatments resulted in significant improvements from baseline in all symptoms of allergic rhinitis as well as in all the domains of the QOL questionnaires. When changes from baseline were compared between treatments, there were no significant differences except in the symptom of nasal congestion, for which pseudoephedrine was more effective than montelukast. Both treatments resulted in a significant increase in NPIF over baseline with no significant difference between treatments. Both drugs were well tolerated with no differences in the tolerability profiles between treatments. Conclusions: Pseudoephedrine and montelukast are equivalent in improving symptoms and QOL and increasing nasal airflow in patients with seasonal allergic rhinitis. The lack of the usual adverse effects in the pseudoephedrine group is ascribed to morning dosing.