Polycythemia vera, illustration. Polycythemia vera is a rare slow-growing blood cancer in which bone marrow makes too many red blood cells. These excess cells thicken blood, slowing its flow. They also cause complications, such as blood clots, which can lead to a heart attack or stroke. Without treatment, polycythemia vera can be life-threatening. Medical care can help ease signs, symptoms and complications of this disease. Over time, in some cases there's a risk of progressing to more-serious blood cancers, such as myelofibrosis or acute leukemia.