The vascular complications in patients with polycythemia vera are microvascular circulatory disturbances typical of thrombocythemia including erythromelalgia, peripheral ischemia, atypical cerebral ischemic attacks, and major arterial and venous thrombotic events. These are positively related to hematocrits due to the increased red cell mass and its concomitant increased whole blood viscosity. Phlebotomy does not prevent the aspirin-responsive microcirculatory circulation disturbances in polycythemia vera because thrombocythemia (platelet count > 400 x 10(9)/L) persists. The risk of major vascular ischemic episodes in poorly controlled polycythemia vera at hematocrits between 0.45 and 0.50 is rather high. The risk of vascular complications in polycythemia vera is best controlled by maintaining the hematocrit at less than 0.45 and the platelet count below 400 x 10(9)/L. The microvascular syndrome associated with thrombocythemia in early stage polycythemia vera in remission by phlebotomy is easily and best controlled by low-dose aspirin (50 to 100 mg) or by selective reduction of platelet count to normal with low-dose myelosuppressive agents. The potential leukemogenic myelosuppressive agents busulfan and hydroxyurea and the nonleukemogenic cytosine interferon-alpha have proven to be effective in the control of the proliferative phase of polycythemia vera. However, data on the natural history of polycythemia vera and the best treatment modality of the various stages of myeloproliferative disease are still lacking.