Background. Thrombotembolic complications are the leading cause of mortality in essential thrombocythemia (ET), but the definition of thrombotic risk remains far from clear. Objectives. The aim of this study was to evaluate the prognostic markers for thrombosis to identify ET patients at risk. Material and methods. Forty-five consecutive patients with ET were studied. This group was divided into two subgroups ET patients with (A) and without (B) history of thrombosis. Each patient has been tested for complete blood count, fibrinogen, factor VIII, D-dimer, protein C, APCR, TAT and F1+2. JAK2 mutation was assessed by RT-PCR. Factor V Leiden and prothrombin genes mutations were screened by DNA sequencing. Results. The median age of ET patients was 62.0 years. JAK2 mutation was found in 24 patients, 21 of them had a history of thrombotic events, and 17/21 were JAK2 positive. Compared to controls, ET patients had a significantly higher WBC and PLT counts, and higher mean platelet volume (MPV), but not Hgb level or RBC count. In ET subgroup A, apart from changes seen in the whole ET, the Hgb level, RBC count, and Hct were also significantly elevated. Interestingly, the MPV was significantly larger in subgroup A, but not in B. Fibrinogen and D-dimers levels were significantly higher in ET group than in controls, but not F1+F2 or TAT. The results of hemostatic tests did not markedly differ between subgroups A and B. APCR was found in 5/45 patients with ET, and 2 out of 5 had a factor V Leiden heterozygous mutation. No prothrombin gene mutation was observed. Conclusions. Our results suggest that MPV can serve as a simple test for assessing the hypercoagulable state in ET patients. It has been confirmed that JAK2 mutation and leukocytosis are independent predictors for thrombotic events in ET patients.