The classic myeloproliferative neoplasms (MPNs) include polycythemia vera and essential thrombocythemia; their molecular basis has been described only recently with the demonstration of recurrent mutations in JAK2 or MPL. While life expectancy may not be significantly shortened, arterial and venous thrombosis constitute the major causes of morbidity and mortality, together with disease evolution to myelofibrosis or transformation to acute leukemia. Therapy is currently aimed at reducing the rate of thrombosis without increasing the risk of hematologic transformation by inappropriate exposure to cytotoxic drugs. Nevertheless, the mechanism(s) finally responsible for the increased thrombotic tendency have not been clearly elucidated, although risk factors for thrombosis have been identified, and are currently employed for stratifying patients to the most appropriate therapeutic options. Abnormalities of blood cells, activation of neutrophils and platelets, and a hypercoagulability state, can all act in conjunction to lead to thrombosis. Intriguing data also point to the JAK2V617F mutation as both a marker and a mechanism for thrombosis. Better knowledge in the pathophysiology of these disorders, and the introduction of molecularly targeted drugs in clinical trials, anticipate the possibility of more specific and efficacious treatment of classic MPN, particularly as concerns the reduction of risk associated with vascular events.