Four patients with primary pulmonary hypertension, microangiopathic hemolysis, and thrombocytopenia were seen at the Charleston (WVa) Area Medical Center between 1983 and 1988. Characteristic laboratory and clinical features of these patients were the following: mild anemia, reticulocytosis, low serum haptoglobin value, microangiopathic red blood cell changes on peripheral blood smear, mild to moderate thrombocytopenia, normal clotting studies (ie, prothrombin time, partial thromboplastin time, fibrinogen), negative direct Coombs' test, negative glucose water test, normal serum urea nitrogen and creatinine levels, lack of improvement of hemolysis and thrombocytopenia in response to vasodilators or calcium channel blockers, and severe plexiform lesions in the pulmonary vasculature with fibrin deposition at autopsy. The hemolysis and thrombocytopenia probably developed as a result of flow through the fibrin deposition in the plexiform lesions in the pulmonary circulation and subsequent shearing of red blood cells and platelets.