Two patients with mesenteric venous thrombosis which occurred late after splenectomy are reported. In the first case the thrombosis was seen, with preceding thrombocytosis, 3 years after splenectomy. In the second case the thrombosis occurred 2 years and 5 months postoperatively. Platelet counts had not been obtained before the mesenteric thrombosis in this case. In both instances persistent severe anaemia for several weeks preceded the symptoms and signs of mesenteric venous thrombosis. Both patients were treated by extended resection of the affected small bowel. Thus, dangerous thrombocytosis may occur in the presence of persistent severe anaemia even late after splenectomy. Thrombocytosis occurring immediately after splenectomy, which is almost inevitable, should be treated with heparin or other anticoagulants. In the late period the avoidance of persistent severe anaemia seems to be important in the prevention of thrombotic complications due to reactive thrombocytosis in splenectomized patients.