Pulmonary microvascular occlusive disease has been investigated using balloon occlusive pulmonary angiography in 31 patients with severe adult respiratory distress syndrome (ARDS) of different origins (14 patients with pneumonia, nine with multiple injury, eight with sepsis). Multiple pulmonary artery filling defects (PAFD) were detected in 13 (42%) patients, with a seven (78%) in nine incidence among those with posttraumatic ARDS. The presence of PAFD did not correlate with the severity of the respiratory failure, with the pulmonary hemodynamic alterations (pulmonary hypertension and increased vascular resistance), or with the final outcome (mortality rate was 54% among patients with PAFD and 61% among those with normal angiograms). These findings suggest that widespread pulmonary microthrombosis is a common event in patients with polytrauma and respiratory failure, with an important pathophysiologic role in the onset of posttraumatic ARDS.