Primary myelofibrosis (PMF) is a rare myeloproliferative bone marrow disorder characterized by clonal myeloproliferation, variable degrees of fibrosis (identified with reticulin or trichrome stain), and abnormal megakaryocyte proliferation resulting in impaired hematopoiesis. It typically occurs in elderly patients (median age is 65 years), although PMF may occur at any age.

The peripheral blood smear often demonstrates leukoerythroblastic changes and teardrop-shaped red blood cells (RBCs), consistent with bone marrow fibrosis. As the marrow is replaced by fibrosis, the spleen will enlarge due to extramedullary hematopoiesis.

The bone marrow biopsy may be a "dry tap" due to extensive fibrosis and/or osteosclerosis replacing the marrow elements. Up to 60% of cases of myelofibrosis contain the JAK2 V617F mutation (which can be detected on peripheral blood or bone marrow).

Myelofibrosis is a chronic condition with many phases. When the disease is more progressed, there is a possibility it may transform into acute leukemia.