This randomized, double-blind, placebo-controlled trial assessed the efficacy of montelukast in the treatment of adults ≥50 years of age with persistent asthma and/or COPD. Primary outcomes included forced expiratory volume in one-second (FEV1) and daytime asthma symptoms scores. Nocturnal symptoms, asthma control, health-related quality of life, peak flow measurements, and health care utilization were also assessed as secondary outcomes. Participants were recruited from the Kaiser Permanente Northwest member population. One hundred forty-nine subjects were randomized to treatment with montelukast (10 mg per day) or placebo, and were followed for a six-week period. No differences in lung function measures, health-related quality of life, health care utilization, and asthma symptom scores were observed; however, the montelukast group had slightly improved asthma control scores compared to the placebo group.