Polycythemia vera (PV) is a rare chronic myeloproliferative disorder in which the bone marrow is characterized by clonal myeloproliferation causing erythrocytosis, increased red blood cell mass, and elevated hemoglobin and hematocrit. PV is associated with an increased risk of thrombotic events and bone marrow fibrosis, and can transform into acute leukemia. It can occur in patients of any age, with a slight male predominance. Median age at diagnosis is 60 years. The condition can begin in childhood and has been associated with mutations in the erythropoietin receptor gene. Rarely, Pacak-Zhuang syndrome may be seen in patients with early-onset polycythemia and paragangliomas.

The peripheral blood smear often demonstrates normochromic, normocytic red blood cells (may be hypochromic and microcytic if iron deficiency is also present), thrombocytosis, and leukocytosis.

The bone marrow may show hypercellularity, megakaryocytic proliferation, fibrosis, and the absence of stainable iron. If the marrow shows fibrosis, the spleen will enlarge due to extramedullary hematopoiesis. More than 95% of cases of PV have a JAK2 mutation (which can be detected on peripheral blood or bone marrow).

PV is a chronic condition with many phases. When the disease is more progressed, there is possibility of increased fibrosis or that it may transform into acute leukemia.