||RATIONALE: A pilot study (Bisgaard H; Study Group on Montelukast and Respiratory Syncytial Virus. A randomized trial of montelukast in respiratory syncytial virus postbronchiolitis. Am J Respir Crit Care Med 2003;167:379-383) reported the efficacy of montelukast in post-respiratory syncytial virus (RSV) bronchiolitic respiratory symptoms. OBJECTIVES: To evaluate the efficacy and safety of montelukast, 4 and 8 mg, in treating recurrent respiratory symptoms of post-RSV bronchiolitis in children in a large, multicenter study. METHODS: This was a double-blind study of 3- to 24-month-old children who had been hospitalized for a first or second episode of physician-diagnosed RSV bronchiolitis and who tested positive for RSV. Patients (n = 979) were randomized to placebo or to montelukast at 4 or 8 mg/day for 4 weeks (period I) and 20 weeks (period II). The primary end point was percentage symptom-free days (%SFD; day with no daytime cough, wheeze, and shortness of breath, and no nighttime cough). MEASUREMENTS AND MAIN RESULTS: No significant differences were seen between montelukast and placebo in %SFD over period I: mean +/- SD for placebo and for montelukast at 4 and 8 mg were 37.0 +/- 30.7, 38.6 +/- 30.4, and 38.5 +/- 29.9, respectively. Least-squares mean differences (95% confidence interval) between montelukast (4 mg) and placebo and between montelukast (8 mg) and placebo were 1.9% (-2.9, 6.7) and 1.6% (-3.2, 6.5), respectively. Secondary end points were similar across treatments. Both doses were generally well tolerated. During the first two treatment weeks, average %SFD was approximately 29%. In post hoc analyses of patients (n = 523) with persistent symptoms (%SFD < or = 30% over Weeks 1-2), differences in %SFD were seen between montelukast and placebo over Weeks 3-24: difference were 5.7 (0.0, 11.3) for montelukast (4 mg) minus placebo and 5.9 (0.1, 11.7) for montelukast (8 mg) minus placebo. CONCLUSIONS: In this study, montelukast did not improve respiratory symptoms of post-RSV bronchiolitis in children.