Platelet-activating factor (PAF) is an inflammatory mediator that provokes neutropaenia, bronchoconstriction and gas exchange defects due to exudation of bulk plasma within the airways. While the inhibitory effects of short-acting beta2-agonists on PAF-induced disturbances have been consistently shown, those of long-acting beta2-agonists are less convincing. To further explore the mechanisms involved in PAF challenge in asthma, 12 patients (forced expiratory volume in one second, 90 +/- 4% predicted) were investigated 2 h after inhaled formoterol (18 microg), in a double-blind, placebo-controlled, crossover design following PAF (18 microg) inhalation. Compared with the placebo, at 5 min, premedication with formoterol reduced PAF-induced cough and dyspnoea, and attenuated increased respiratory system resistance (by 67%) and arterial deoxygenation (by 50%). Likewise, ventilation-perfusion (V'A/Q') inequality improved, as reflected by the dispersion of pulmonary blood flow (by 63%) and an overall index of V'A/Q' heterogeneity (by 71%). In contrast, PAF-induced facial flushing, neutropaenia and subsequent rebound neutrophilia remained unchanged. The improvement in gas exchange abnormalities shown after platelet-activating factor in patients with asthma pretreated with formoterol at the recommended clinical dose may reflect, in addition to its class effects, an anti-exudative effect of formoterol in the airways.