Clinical observations, coagulation studies, and analysis of simultaneously obtained samples of lung effluent and arterial or venous blood for hematocrit and protein composition as determined by gel filtration were performed on 15 infants collapsing with blood stained liquid flowing from their tracheas. The results indicate that (1) The lung effluent was in most cases hemorrhagic edema fluid and not whole blood. (2) The most important precipitating factor was probably acute left ventricular failure due to asphyxia, although other factors favoring increased filtration of liquid from pulmonary capillaries or causing lung damage may have played a contributory role in pathogenesis. (3) Coagulation disorders probably served to exacerbate the condition but not initiate it.