Introduction: In numerous studies of myeloproliferative neoplasms (MPN): polycythemia rubra vera (PRV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), the incidence of the V617F mutation in the JAK2 gene has been observed. Objectives: The aim of this research is to evaluate the frequency of the presence of somatic mutation V617F in the JAK2 gene in the groups of patients with MPN and to examine whether there is a difference in the expression of the mutation between different subtypes of MPN. Participants and Methods: The group of participants includes 64 patients with MPN diagnoses: PRV (34), ET (26) and PMF (4). The DNA was isolated from a blood samples and analysis of JAK2 V617F was performed by quantitative real-time PCR. Results: Somatic mutation of the V617F JAK2 gene was present in most patients (P <0.001), in 56 of them (87.5%). A significant correlation was found between JAK2 status and the diagnosis of the subjects (P = 0.03). The frequency of mutation in PMF is 100%, in PRV 97.06%, and in ET 73.08%. 17.2% of patients had thrombosis (P <0.001). Of that number, 72.7% were arterial thrombosis. The mean value was significantly higher in subjects with the JAK2 mutation present for the following clinical parameters; E, Hb, Hct, L (P <0.05). The value of Hb is significantly higher, and the value of LDH is significantly lower in subjects with present JAK status (P <0.05). Conclusion: In published studies conducted in Croatia and in other populations, data for the frequency of JAK2 V617F mutation in patients with PRV, ET and PMF vary. Our results for the PRV and ET subtypes are within the range of mutation frequencies present in published studies. The number of patients included in the PMF is too small to draw final conclusions based on statistical analysis.