Background: Myeloproliferative neoplasms (MPNs) are characterized by an increased frequency of thrombotic complications. Patients with MPNs show little or no abnormalities in traditional coagulation tests. Chronic inflammation associated with pathological secretion of several inflammatory cytokines leads to activation of endothelial cells and imbalance between coagulation and anticoagulation systems. Aims: To estimate the natural anticoagulants activity and activated protein C resistance in MPNs patients. Methods: The study included 99 MPNs patients (aged 25 – 86 years, median = 55) and 43 healthy controls (aged 30-73 years, median = 46). There were 65 female and 34 male in the study group. 75 (76%) patients were JAK2V617F positive, 24 (24%) were JAK2V617F negative. Antithrombin (AT), protein C (PC) activities and free protein S level (PS) estimation was performed by standard techniques. Thrombin generation was assessed by calibrated automated thrombinography (CAT) according to Hemker et al. Measurement was conducted in platelet poor plasma with or without thrombomodulin (TM). Such parameters as endogenous thrombin potential (ETP, nM∗min) and peak thrombin (Peak, nM) were evaluated. ETP and Peak sensitivity for TM were calculated as percent of these parameters decrease after TM adding to assay (S ETP, % and S Peak, % respectively). STATISTICA 6.0 package was used for data analysis. Parameter results were presented as median (Me) with 95% confidence intervals (CI), p