To determine the possible role of platelet-activating factor (PAF) in pulmonary embolism (PE), the reactivity of the airways to inhaled PAF, using doses ranging from 6.25 to 400 micrograms, was examined in 24 patients with a past episode of PE. Twelve of these patients had experienced acute respiratory insufficiency during the episode (with or without additional symptoms). None of the remaining 12 patients had experienced any respiratory symptoms during the PE episode. Diagnosis was established by means of a ventilation-perfusion lung scan performed when admitted to the hospital because of deep venous thrombosis (DVT) in the lower limbs. Nonspecific bronchial reactivity as measured by the response to bronchoprovocation testing with methacholine showed no significant differences between both groups. PAF caused a dose-dependent bronchoconstriction defined by at least a 35 percent decrease in specific airway conductance (SGaw) in all patients. The average dose of PAF needed to decrease SGaw 35 percent was significantly lower in patients who had had a symptomatic PE than in those with asymptomatic PE (p = 0.011). This finding suggests that patients who suffered from symptomatic PE may present a greater airway reactivity to inhaled PAF. This different behavior might explain the existence of some of the respiratory symptoms of PE, which could be attributed to PAF-related effects. However, additional studies are needed to evaluate the role of PAF in PE.