Erythrocytosis (increased RBC mass or polycythemia) maybe primary due to an intrinsic defect in the bone marrow stem cells/erythroid progenitor cells or secondary where the red cell production is driven by factors external to the erythroid compartment, such as increased erythropoietin production for any reason. Primary and secondary causes can be further classified into congenital and acquired causes. The classical acquired primary erythrocytosis is Polycythemia Vera where patients have an abnormal clone and frequently an increased white blood cell and platelet count. Majority of patients have a clone of cells with gain of function mutations in JAK2. Congenital causes include mutations of the erythropoietin receptor and defects in oxygen sensing pathways including VHL, PHD2 AND HIF2A mutations.