Several international working groups cooperated to propose new diagnostic guidelines for polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF) to the steering committee of the World Health Organization. Because JAK2 mutation status presents a decisive diagnostic test in PV, this feature was introduced as a major criterion. Minor criteria, such as characteristic bone marrow morphology, low erythropoietin level, and erythroid colony formation, were kept as supporting parameters. In PMF, major diagnostic criteria were established by histologic features independent of the presence of relevant fibrosis or myelofibrosis with myeloid metaplasia. JAK2 mutation status was restricted to positive findings to exclude reactive myelofibrosis. A decrease in the platelet level was proposed for ET, because vascular complications may occur at lower platelet counts. As with PMF, morphology plays a distinctive role in diagnosis, particularly for its differentiation from early-stage PV or prodromal PMF associated with thrombocytosis.