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This review focuses on polycythemia vera (PV)-its diagnosis, cellular and genetic pathology, and management. In Section I, Dr. Pearson, with Drs. Messinezy and Westwood, reviews the diagnostic challenge of the investigation of patients with a raised hematocrit. The suggested approach divides patients on their red cell mass (RCM) results into those with absolute (raised RCM) and apparent (normal RCM) erythrocytosis. A standardized series of investigations is proposed for those with an absolute erythrocytosis to confirm the presence of a primary (PV) or secondary erythrocytosis, with abnormal and normal erythropoietic compartments respectively, leaving a heterogenous group, idiopathic erythrocytosis, where the cause cannot be established. Since there is no single diagnostic test for PV, its presence is confirmed following the use of updated diagnostic criteria and confirmatory marrow histology. In Section II, Dr. Green with Drs. Bench, Huntly, and Nacheva reviews the evidence from studies of X chromosome inactivation patterns that support the concept that PV results from clonal expansion of a transformed hemopoietic stem cell. Analyses of the pattern of erythroid and myeloid colony growth have demonstrated abnormal responses to several cytokines, raising the possibility of a defect in a signal transduction pathway shared by several growth factors. A number of cytogenetic and molecular approaches are now focused on defining the molecular lesion(s). In the last section, Dr. Barbui with Dr. Finazzi addresses the complications of PV, notably thrombosis, myelofibrosis and acute leukemia. Following an evaluation of published data, a management approach is proposed. All patients should undergo phlebotomy to keep the hematocrit (Hct) below 0.45, which may be all that is required in those at low thrombotic risk and with stable disease. In those at high thrombotic risk or with progressive thrombocytosis or splenomegaly, a myelosuppressive agent should be used. Hydroxyurea has a role at all ages, but (32)P or busulfan may be used in the elderly. In younger patients, interferon-alpha or anagrelide should be considered. Low-dose aspirin should be used in those with thrombotic or ischemic complications.