Polycythemia vera is a disease in which too many red blood cells are made in the bone marrow.
In polycythemia vera, the blood becomes thickened with too many red blood cells. The number of white blood cells and platelets may also increase. These extra blood cells may collect in the spleen and cause it to swell. The increased number of red blood cells or platelets in the blood can cause bleeding problems and make clots form in blood vessels. This can increase the risk of stroke or heart attack. In patients who are older than 65 years or who have a history of blood clots, the risk of stroke or heart attack is higher. Patients also have an increased risk of developing acute myeloid leukemia or primary myelofibrosis.