The primary objective of this study was to determine whether montelukast, an oral leukotriene receptor antagonist, provides additional clinical benefit to the effect of inhaled corticosteroids. A total of 642 patients with chronic asthma (FEV(1) 50 to 85% of predicted value and at least a predefined level of asthma symptoms) incompletely controlled with inhaled beclomethasone, 200 microg twice daily using a spacer device, during the 4-wk run-in period were randomly allocated, in a double-blind, double-dummy manner to one of four treatment groups: (1) montelukast 10 mg plus continuing inhaled beclomethasone; (2) placebo tablet plus continuing inhaled beclomethasone; (3) montelukast 10 mg and inhaled placebo (after blind beclomethasone removal); and (4) placebo tablet and inhaled placebo (after blind beclomethasone removal). The primary endpoints were FEV(1) and daytime asthma symptoms score. Montelukast provided significant (p < 0.05) clinical benefit in addition to inhaled beclomethasone by improving FEV(1), daytime asthma symptom scores, and nocturnal awakenings. Blind removal of beclomethasone in the presence of placebo tablets caused worsening of asthma control, demonstrating that patients received clinical benefit from inhaled corticosteroids. Blind removal of beclomethasone in the presence of montelukast resulted in less asthma control but not to the level of the placebo group. All treatments were well tolerated; clinical and laboratory adverse experiences were generally similar to placebo treatment in this study. In conclusion, montelukast provided additional asthma control to patients benefitting from, but incompletely controlled on, inhaled beclomethasone.