Two hundred thirty-one consecutive Chinese patients with ET were identified from the patient record systems in 5 regional hospitals in Hong Kong. Patient records were reviewed for demographic data, initial symptoms, treatment, and subsequent clinical course, including thrombosis, bleeding, and conversion to MF, PV, or AML. Diagnosis of ET was based on the revised Polycythemia Vera Study Group5 criteria (Table 1), including persistent thrombocytosis with a platelet count higher than 600×103/μL and exclusion of reactive causes, MDS, Philadelphia chromosome–positive chronic myeloid leukemia, and other Philadelphia chromosome–negative myeloproliferative disorders (by hematocrit reading <40%, normal marrow iron store, and absent or minimal marrow fibrosis). In addition to evaluation of marrow iron, iron status was assessed by serum ferritin and serum iron levels and by total iron-binding capacity. The presence of the Philadelphia chromosome was detected by karyotyping, fluorescence in situ hybridization, or reverse transcription–polymerase chain reaction.14- 15 A diagnosis of subsequent conversion to MF was based on the presence of marrow fibrosis and splenomegaly, a leukoerythroblastosis, and teardrop red cells, in addition to exclusion of secondary causes of MF.16- 17 The diagnoses of AML and MDS were made according to the French-American-British classification.18- 19 Recently, a prefibrotic phase of MF has been proposed that has a specific megakaryocyte morphologic structure, demonstrates a markedly increased propensity to MF, and is associated with inferior survival.20 For patients who developed MF, diagnostic bone marrow aspirates were reviewed to identify the characteristic morphologic features of abnormal megakaryopoiesis (grouping of cells containing compact bulbous nuclei, accompanied by bizarre features of differentiation).21 The initial treatment was according to the discretion of the attending physicians. In general, hydroxyurea would be started if patients were symptomatic or were initially seen with a platelet count higher than 1000×103/μL. The target platelet count was lower than 600×103/μL. Low-dose aspirin (100 mg/d) therapy would be started if there was a history of arterial events, including ischemic heart disease, thrombotic strokes, transient ischemic attacks, peripheral vascular disease, and erythromelalgia, or a history of venous thrombosis.