[en] This study compared the acute bronchial and hematologic effects of inhaled platelet activating factor (PAF) (30 micrograms as a single dose) in 19 patients with mild asthma and 19 normal subjects. Each subject underwent a methacholine bronchial challenge 1 week before PAF challenge to determine the concentration of methacholine causing a 20 percent fall in FEV1 (PC20M). On the day of PAF challenge, specific conductance (SGaw), FEV1, FEF25-75, and platelet and leukocyte counts were measured before, and 5, 10, 15, and 20 min after PAF inhalation. Changes in pulmonary and hematologic parameters were expressed as percent of control (saline solution/ethanol solution). Unlike normal subjects, subjects with asthma had bronchial hyperresponsiveness to methacholine: geometric mean (range): 0.59 mg/ml (0.07 to 9.8) vs > 32 mg/ml. Acute bronchial obstruction over the first 20 min after PAF inhalation was more pronounced in asthmatics than in normal subjects whatever the functional index considered (p < 0.01). In asthmatics (n = 19), mean (SEM) maximal fall in SGaw, FEV1, and FEF25-75 reached 50 percent (6), 11 percent (4), and 19 percent (5), respectively, while in normal subjects (n = 19) the maximal decreases were 24 percent (6), 4 percent (1), and 6 percent (1), respectively. In asthmatics, no correlation was found between log PC20M and log fall in FEV1 after PAF (r = 0.04 p > 0.05). In asthmatics and normal subjects, inhaled PAF caused a transient fall in neutrophils and monocytes by 5 min followed by a full recovery at 15 min and 20 min. These hematologic changes were not significantly different between the two groups. While not correlated with their airway responsiveness to methacholine, asthmatics, compared with normal subjects, develop an exaggerated acute airway obstruction in response to PAF. In contrast, hematologic changes induced by PAF do not differ between asthmatics and normal subjects.