Polycythemia vera is an abnormal increase in blood cells, primarily red blood cells (polycythemia) but also white blood cells and platelets. As numbers of blood cells rise, the blood becomes thicker, increasing the risk of dangerous blood clots (thrombosis).

The cause of polycythemia vera is not known. Most often, this rare disease does not cause medical problems. In some cases, the cell growth increases and can cause leukemia.

Symptoms include a headache, itchiness, and a reddish face.

Treatment may include removal of blood (phlebotomy) on a regular basis, such as weekly, until the blood cell count is normal. Medicine might be used to slow down the production of red blood cells.