Increased angiogenic activity has been demonstrated in myelofibrosis with myeloid metaplasia (MMM), chronic myeloid leukemia (CML), and essential thrombocythemia (ET) by both bone marrow microvessel density evaluation and measurement of circulating angiogenic factors. MMM is probably the disease with the more pronounced angiogenesis among myeloproliferative disorders but the significance of this finding remains speculative since the angiogenic activity is not correlated with any of the clinical and laboratory features of the disease. Circulating serum levels of angiogenic factors such as vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) were found increased in MMM, CML and ET but the frequent thrombocytosis that accompanies these diseases could limit the interpretation of these data since platelets and megakaryocytes may be considered a major source at least for VEGF. However, CML patients treated with interferon were found to have lower VEGF and HGF levels than untreated or hydroxyurea-treated patients, thus suggesting a possible antiangiogenic mechanism of this drug. In addition, preliminary experiences with the antiangiogenic drug thalidomide have shown therapeutic activity in some myeloproliferative disorders.