The three main chronic myeloproliferative disorders are polycythemia vera (PV), essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF). These are clonal neoplastic diseases characterized by proliferation of one or more hematopoietic lineages. Recently a mutation of the Janus Kinase 2 (JAK2) gene that leads to the substitution of phenylalanine for valine at position 617 of the JAK2 protein, JAK2 V617F, has been found in 76% to 97% of patients with PV, 29% to 57% of patients with ET and 50% of patients with IMF. This mutation confers constitutive activity on to the JAK2 protein and appears to play an important role in the pathobiology of these conditions. However, not all patients with myeloproliferative disorders have this mutation and it may not be the primary cause of these diseases. The primary goal of this prospective natural history study is to investigate the molecular basis of these diseases in groups of patients who have JAK2 V617F and in those who do not. A second goal is to identify biomarkers for PV and the other myeloproliferative disorders that are easier to measure than JAK2 V617F. Approximately, 150 patients with myeloproliferative disorders will be studied over 3 years. The studies will involve the collection of 40 mL to 50 mL of peripheral blood from each subject. The blood will be used to assess neutrophil gene and protein expression, gene polymorphisms, and plasma protein levels.