Polycythemia (or polycythaemia or erythrocytosis) is a conition in which there is a net increase in the total number of blood cells, primarily red blood cells, in the body. The overproduction of red blood cells may be due to a primary process in the bone marrow (a so-called myeloproliferative syndrome), or it may be a reaction to chronically low oxygen levels or, rarely, a malignancy.