The involvement of platelet-activating factor (PAF) in bronchial hyperresponsiveness (BHR) in bronchial asthma has been controversial. To determine whether PAF is involved in BHR in humans, we carried out a randomized, double-blind, placebo-controlled, two-phase cross-over study on the effects of Y-24180, a potent, specific, orally active PAF receptor antagonist, on BHR to methacholine in patients with asthma. The subjects were 13 patients with extrinsic stable asthma. The provocative concentration of methacholine producing a 20% fall in FEV1 (PC20-FEV1) was measured as an index of BHR. Y-24180 (20 mg twice a day) or a placebo was orally administered for 2 wk, respectively. At the time of cross-over from the first treatment regimen to the second regimen, administration of the test drug was suspended for 2 wk. The methacholine challenge test was performed four times, before and after the first treatment period and before and after the second treatment period. Compared with the placebo, Y-24180 significantly (p = 0.005) improved the PC20-FEV1 value without carryover effect and period effect by analysis of variance. These results suggest that PAF is an important mediator involved in the BHR of bronchial asthma in humans.