Primary myelofibrosis is a myeloproliferative disorder, characterized typically by pancytopenia, splenomegaly, a leukoerythroblastic blood smear, fibrosis on the bone marrow examination, and myeloid metaplasia. Like all myeloproliferative disorders, it is due to somatic mutation and clonal proliferation of hematopoietic progenitors. Fibrosis occurs secondary to cytokines secreted by malignant cells. These patients are symptomatic because of cytopenias. Bone marrow failure and transformation to acute myeloid leukemia is the major reason for mortality in these patients. Elevated blood levels of hematopoietic progenitors (identified by positivity for the CD34 antigen) are well documented and correlates with prognosis. Therefore two major identifying features of primary myelofibrosis as opposed to secondary fibrosis of the marrow are: 1) clonal hematopoiesis and 2) high CD 34 positive hematopoietic precursors.