We studied 25 patients with myelofibrosis with myeloid metaplasia and 19 patients with secondary myelofibrosis associated with pulmonary hypertension (PH). In these 2 groups, we compared the peripheral-blood CD34 count, the clonality of granulocytes and platelets in peripheral blood, the mutational status of the JAK2 kinase gene, and the morphology of the peripheral blood and bone marrow. We found that the following were distinctive features of myelofibrosis with myeloid metaplasia but not of secondary myelofibrosis due to PH: high circulating CD34 cell count, the presence of clonal platelets and granulocytes and of peripheral-blood dacrocytes, and a JAK2 1849G>T (V617F) mutation. We conclude that these are intrinsic features of clonal progenitors present in patients with myelofibrosis due to myeloproliferative disorders and that these features are not due to the abnormal marrow architecture seen in secondary myelofibrosis. © 2006 by The American Society of Hematology.