In primary myelofibrosis, a progressive scarring (fibrosis) of the bone marrow occurs. As a result, blood forms in sites other than the bone marrow, such as the liver and spleen. This causes an enlargement of these organs. The cause and risk factors are unknown. It commonly occurs in the spent phase of Polycythemia Rubra Vera, possibly in response to the medication hydroxyurea poisoning the marrow.