Erythrocytosis is a condition in which the number of red blood cells, which carry oxygen, is higher than normal. Thrombocytosis is a condition in which the number of platelets, which are the cells that help blood to clot, is higher than normal. Erythrocytosis and thrombocytosis can be the result of another disease process or diseases called polycythemia vera and essential thrombocythemia. Polycythemia vera and essential thrombocythemia may be associated with such problems as blood clots, bleeding, or serious bone marrow diseases, including some types of leukemia. However, many people with polycythemia vera and essential thrombocythemia never develop problems, and there is uncertainty about the best way to treat these diseases. Studies that have used information from hospital discharge records or cancer registries suggest that polycythemia vera and essential thrombocythemia are rare, occurring in only 1 to 5 of every million people. However, no studies have examined the frequency of these conditions in the general population.