A patient with an 11 year history of paroxysmal nocturnal hemoglobinuria presented with severe abdominal pain. On admission, the hematocrit value was 30 per cent and unchanged from repeated measurements during the previous three years. Abdominal angiography identified extensive thromboses of the splenic and portal venous systems. After initial improvement on heparin therapy, the patient experienced additional abdominal crises. A ruptured and multifragmented spleen was removed at the time of exploratory laparotomy. Postoperatively, after a several days' interval of improvement, the patient experienced additional thrombotic episodes of the abdomen, upper extremities and cerebral cortex. The latter was associated with disabling nerve paralysis. With continuous intravenous heparin plus steroid therapy, the patient's condition improved progressively. Despite the numerous thrombotic episodes during the prolonged hospital course, no hemolytic episodes were observed. This is the first report of documented splenic rupture in a patient with paroxysmal nocturnal hemoglobinuria.