On the basis of a series of clinical investigations, our present viewpoints on posttraumatic pulmonary insufficiency due to the microembolism syndrome may be summarized as follows. Two forms of this syndrome exist: 1. An early microembolism syndrome caused by transient deposition of fibrin-rich microemboli in the pulmonary microcirculation, giving rise to a temporarily low ventilation/perfusion ratio. This pulmonary reaction pattern is often subclinical. 2. A delayed microembolism syndrome caused by persistent fibrin-rich microemboli due to fibrinolysis inhibition, giving rise to increased vascular permeability and progressive interstitial and alveolar oedema. This pulmonary reaction pattern leads to pulmonary insufficiency with the characteristic radiographic changes and clinical symptoms of respiratory distress. Whether the early syndrome will develop into the delayed form may depend mainly upon the severity of the trauma, the capacity of the fibrinolytic system to clear the lungs, the form of treatment given, and the cardiopulmonary state of the patients.