Factors related to the development of acquired von Willebrand syndrome in patients with essential thrombocythemia and polycythemia vera. 
We characterized acquired von Willebrand syndrome (AVWS) among essential thrombocythemia (ET) and polycythemia vera (PV) patients.  A review of patients with ET or PV evaluated for AVWS.  Of 116 patients with ET, 64 (55%) developed AVWS; of 57 with PV, 28 (49%) developed AVWS.  Median platelet counts of ET and PV patients who developed AVWS were 920×109/L and 679×109/L, respectively (P=0.01).  Of patients who developed AVWS, 69.5% had platelet counts below 1000×109/L. Bleeding was more common in patients with AVWS, among both ET and PV patients (P<0.001).  VWF:RCo levels and VWF:RCo/VWF:Ag ratio were lower among JAK2 V617F positive- vs. JAK2 V617F negative- ET patients (P=0.02 and P=0.002, respectively); whereas VWF:Ag levels were comparable (P=0.96).  ET patients harboring the JAK2 V617F mutation were more likely to develop AVWS than were calreticulin-positive patients (70.3% vs. 45.7%, P=0.02), despite lower platelet counts (median 773 vs. 920×109/L, P=0.05).  In multivariable analysis, younger age (P=0.002), platelet count (P<0.001), hemoglobin level (P=0.01) and JAK2 V617F mutation (P=0.01) independently predicted the development of AVWS among ET patients; whereas only platelet count predicted its development among PV patients (P<0.001).  Among ET and PV patients, AVWS was common and associated with higher bleeding rates and higher platelet count; nonetheless, most AVWS patients had platelet counts under 1000×109/L. Thus, AVWS screening should be included in routine assessment of ET and PV patients.  Among ET patients, JAK2 V617F was a main driver for the development of AVWS.