Many patients with acute asthma do not respond adequately to currently accepted therapy, including oxygen, beta-agonists, and corticosteroids. Leukotriene receptor antagonists such as montelukast have demonstrated efficacy in chronic asthma, but their efficacy in acute asthma is unknown. In this randomized, double-blind, parallel-group pilot study, adults with moderate to severe acute asthma received standard therapy plus either intravenous montelukast (7 or 14 mg) or matching placebo. A total of 201 patients were randomized, and 194 had complete data available for analysis. There was no difference in FEV1 response between the 7- and 14-mg montelukast groups. Montelukast improved FEV1 over the first 20 minutes after intravenous administration (mean percentage change from prerandomization baseline, 14.8% versus 3.6% for the pooled montelukast and placebo treatment groups, respectively; p = 0.007). This benefit was observed at 10 minutes and over 2 hours after intravenous therapy. Patients treated with montelukast tended to receive less beta-agonists and have fewer treatment failures than patients receiving placebo. The tolerability profile for montelukast was similar to that observed for placebo, and no unexpected adverse experiences were observed. We conclude that intravenous montelukast in addition to standard therapy causes rapid benefit and is well tolerated in adults with acute asthma.