Background: A nasal spray containing the antiallergy agent olopatadine hydrochloride is being developed for the treatment of seasonal allergic rhinitis (SAR). Objective: To evaluate the safety and efficacy of 2 concentrations of olopatadine nasal spray vs placebo in patients with SAR. Methods: This was a multicenter, randomized, double-blind, placebo-controlled study. After a 3- to 21-day placebo run-in, 565 patients aged 12 to 80 years were randomized to receive 0.4% or 0.6% olopatadine or placebo, 2 sprays per nostril twice daily for 2 weeks. Patients evaluated morning and evening reflective and instantaneous nasal symptoms (sneezing, stuffy nose, runny nose, and itchy nose, which compose the total nasal symptom score [TNSS]) and ocular symptoms and completed the Rhinoconjunctivitis Quality of Life Questionnaire (RQLQ). Results: Olopatadine spray (0.4% and 0.6%) was significantly superior to placebo for percentage change from baseline in overall reflective (P = .004 and P < .001, respectively) and instantaneous (P = .02 and P = .003, respectively) TNSSs. Also, 0.6% olopatadine was significantly superior to placebo for reducing the reflective and instantaneous assessments of sneezing runny and itchy nose, and itchy eyes; the instantaneous assessments of watery eyes; and the overall and all 7 domain scores of the RQLQ (P < .05). Olopatadine spray exhibited a safety profile comparable with that of placebo. Conclusions: Olopatadine nasal spray (0.4% and 0.6%) provided statistically significant improvements in allergic rhinitis symptoms compared with placebo regarding TNSSs (reflective and instantaneous) and in quality-of-life variables in patients with SAR. Olopatadine nasal spray administered twice daily was safe and well tolerated in adolescents and adults.