We used the thrombin generation assay to evaluate the hypercoagulable state according to JAK2(V617F) mutational status in essential thrombocythemia (ET) and polycythemia vera (PV) patients. Thrombin generation was determined in the presence and absence of activated protein C (APC), and APC resistance was expressed as normalized APC sensitivity ratio (nAPCsr). Tissue factor pathway inhibitor (TFPI), total and free protein S (PS), prothrombin (FII), factor V (FV), and neutrophil elastase were measured in plasma; CD11b was measured on neutrophils. Compared with normal controls, patients had a lower endogenous thrombin potential in the absence of APC but had a higher endogenous thrombin potential in the presence of APC, showing the occurrence of APC resistance. The nAPCsr increased in JAK2(V617F) carriers compared with noncarriers and was highest in JAK2(V617F) homozygous patients. FII, FV, free PS, and TFPI levels were reduced in patients, mainly in JAK2(V617F) carriers. Multiple regression analysis indicated the low free PS level as major determinant of the increased nAPCsr. Elastase was increased in patients and inversely correlated with free PS. In conclusion, these data indicate the occurrence of acquired APC resistance in ET and PV patients, probably because of a reduction in free PS levels. The APC-resistant phenotype is influenced by the JAK2(V617F) mutational load.