The aim of this double-blind, multicenter trial was to determine the duration of bronchodilating activity of a single 4-mg dose of azelastine in chronic asthmatics. Eligible patients had an FEV1 of 40-79% of predicted value with demonstrated reversibility of airway obstruction. A single dose of azelastine 4 mg resulted in rapid onset of bronchodilating activity with a 12-hr duration of action as demonstrated by a clinically and statistically significant mean percent improvement in FEV1, from hours 2-12. As an overall measure of improvement in FEV1, the area under the curve was statistically significantly greater for azelastine than for placebo. Mean improvements in symptom severity in the azelastine group exceeded those for placebo at all observation points. No serious adverse experiences were associated with azelastine.