The safety, tolerability and bronchodilator properties of inhaled verlukast (MK-0679), a new potent and selective LTD4-receptor antagonist, were studied in 12 asthmatic subjects with more than 15% increase in FEV1 after salbutamol inhalation. On three separate study days the patients inhaled placebo, verlukast 2 mg and verlukast 8 mg from a metered dose inhaler according to a randomized, double-blind, cross-over allocation schedule. Pulmonary function and tolerability were assessed regularly and after 8 h a second dose of test drug was inhaled. Thirty minutes later a beta 2-agonist dose-response curve was performed by inhaling salbutamol in cumulative doses of 200, 400 and 800 micrograms. Verlukast (8 mg) caused significant improvement in mean FEV1 from 1.5 through 8 h after inhalation as compared to placebo (P less than 0.05). The maximum change in FEV1 occurred at 2 h after inhalation with mean percent increases above baseline of 3.5, 7.7, and 9.2% after placebo, verlukast 2 mg and 8 mg, respectively. The bronchodilator response to inhaled salbutamol was significantly larger after verlukast 8 mg than after placebo pretreatment (P less than 0.05), whereas verlukast 2 mg afforded no additive bronchodilator effect. We conclude that inhalation of the LTD4-antagonist verlukast induces modest but significant bronchodilatation and may be beneficial in the treatment of asthma.