Clinical and autopsy studies have shown an association between pulmonary microembolism and acute respiratory failure after trauma or sepsis. Prophylaxis and treatment with the aim of decreasing the fibrin deposition in the lungs were associated with a decrease in the incidence and death rate of this syndrome. Small fibrin degradation products (peptides) are accumulated in the lungs and are only slowly cleared from this organ, especially during states of fibrinolysis inhibition. These peptides may contribute to the pulmonary damage in several ways. They act by interfering with other vasoactive substances as bradykinin, histamine and products of the arachidonic acid cascade. Products of the cyclooxygenase pathways as thromboxane A2 play a major role in early microembolism whereas lipoxygenase products seem to be involved in delayed microembolism. Pulmonary microembolism thus seems to be one important, but certainly not the only pathogenetic factor in acute "idiopathic" respiratory failure. Other factors such as pulmonary contusion, aspiration of gastric contents or blood, or oxygen toxicity, might well be contributory in some cases. Pulmonary microemboli containing fibrin and leukocytes are probably also involved as contributory agents in some cases in the large group of acute respiratory failure due to "known factors".