Platelet-activating factor (PAF) is an inflammatory mediator capable of inducing protracted inflammation of the airways and bronchial hyperreactivity. Twenty-one asthmatic children were evenly divided into three groups and each group performed a double-blind, placebo-controlled and crossover study on the effect of aerosolized BN52021, a PAF antagonist, on the bronchoconstriction induced by PAF, methacholine, or specific allergen, respectively. One group of healthy children was included for comparison. Total WBC, neutrophils, and eosinophils were counted before and after PAF challenge. The results showed the following: (1) six of seven asthmatics and one of seven normal subjects gave a positive bronchial provocation with PAF; (2) in asthmatics, prior inhalation of BN52021 could inhibit the bronchoconstriction induced by PAF (6/6) and allergen (3/7), but not by methacholine; and (3) 5 min after inhalation of PAF, there was a marked decrease of peripheral blood eosinophils and neutrophils that could be inhibited by prior inhalation of BN52021 in normal subjects but not in asthmatics. These findings support the idea that PAF may be involved in the pathogenesis of bronchial asthma and PAF antagonist may have a role in the prevention and treatment of this disease.