Pulmonary vascular obstruction is most likely to occur in patients with the severest form of ARDS. Obstruction has been ascribed to fibrin thrombosis and to activation of the blood coagulation system, both well-documented in ARDS. The obstruction has been postulated to occur in two stages during post-traumatic ARDS: an initial transient pulmonary vasoconstriction, attributable to platelet and leukocyte aggregation and the release of vasoactive substances; and, two to five days post injury, a more malignant "delayed microembolism" stage, attributable to the development of protein-rich edema and fibrin deposition in the lungs. Bedside balloon occlusion pulmonary angiography, in a single exposure, allows identification of intraluminal filling defects and occlusions of arteries greater than or equal to 1.0 mm in diameter, irregularity and decreased filling of side branch arteries 0.5 to 1.0 mm in diameter, and decreased filling of the pulmonary microvascular background. Plain radiography findings correlate poorly with angiography results, but the mortality rate of patients who exhibit angiographic evidence of obstruction is twice as high as it is in patients with normal angiography.