Azelastine is an orally effective inhibitor of mediator activity in allergic reactions and has also been demonstrated to have bronchodilator activity. In this randomized, double-blind, placebo-controlled, multicenter study, 150 patients, aged 12 to 60 years, with moderate to severe asthma, received a single oral dose of 2, 4, 8, 12, or 16 mg of azelastine or placebo. Theophylline was stopped 24 hours and other bronchodilators at least 8 hours before the study day. Patients were evaluated for 8 hours after dose by spirometry and were monitored for adverse effects. All doses of azelastine produced bronchodilation with 4 mg greater than 2 mg greater than placebo; higher doses did not increase magnitude or duration of effect. We conclude that azelastine produces significant bronchodilation of long duration. The optimal dose appears to be 4 mg for adolescent and adult patients with asthma.