polycythemia (pŏlˌēsĪthēˈmēə) [key], condition characterized by an increase in the production of red blood cells, or erythrocytes, in the blood. Primary polycythemia, also called erythremia, or polycythemia vera, is a chronic, progressive disease, most common in middle-aged men. It is characterized by overgrowth of the bone marrow, abnormally increased red blood cell production, and an enlarged spleen. Symptoms, include headache, inability to concentrate, and pain in the fingers and toes. There is a danger of blood clotting or hemorrhage (see thrombosis). Primary polycythemia is treated by radiation, periodic removal of some blood (phlebotomy), or chemotherapy with antimetabolite drugs, e.g., Cytoxan. In secondary polycythemia, or erythrocytosis, the proliferation of red blood cells results from the body's attempt to compensate for other conditions, such as prolonged lack of oxygen at high altitudes or chronic lung or heart insufficiency. Certain tumors are also associated with increased red blood cell production. In secondary polycythemia the treatment is directed toward the underlying cause.