Our experience with acute hemorrhagic ulcerations of the gastrointestinal tract indicates that the lesions have a thrombotic etiology. The deposition of fibrin thrombi in the microcirculation of the mucosa and submucosa was recognized in 62 of 89 patients. This interpretation is supported by the clinical coagulation studies which were suggestive that an episode of intravascular clotting had occurred in two thirds of the cases. The lesions are frequently multiple and superficial and primarily involve the mucosa. They do not conform to the geographic distribution of the large vessels of the bowel. This pattern of infarct necrosis can be explained by the localization of fibrin thrombi in the capillaries and small vessels of the mucosa and submucosa during an episode of disseminated intravascular coagulation. Of particular interest is the association of microvascular thrombosis with pseudomembranous enterocolitis.