Primary myelofibrosis is myelofibrosis that is diagnosed without any preceding myeloproliferative neoplasm. Secondary myelofibrosis refers to the bone marrow fibrosis that sometimes forms in people who were first diagnosed with polycythemia vera or essential thrombocythemia. Secondary myelofibrosis is sometimes referred to as a fibrotic phase of essential thrombocythemia or polycythemia vera. Alternatively, secondary myelofibrosis is called post-PV myelofibrosis or post-ET myelofibrosis to indicate the disease that had transformed to the fibrotic phase. Primary and secondary myelofibrosis have many of the same complications and treatments.