Hemostatic disorders are a major clinical problem in patients with myeloproliferative neoplasms (MPNs) and they are the second most common cause of death in MPN patients, after infections. The aim of this study was to assess the fibrinolytic potential of the blood of patients with MPNs. The study involved 112 patients with MPNs diagnosed at the Hematology Clinic Dr J. Biziel University Hospital No. 2 in Bydgoszcz, Poland. The study group included 63 patients with essential thrombocythemia, 29 with polycythemia vera, 11 with chronic myelogenous leukemia (CML) and nine with primary myelofibrosis. The control group consisted of 25 healthy volunteers who were age and sex-matched. The following parameters were determined: concentration of tissue plasminogen activator antigen (t-PA:Ag), plasminogen activator inhibitor type 1 antigen concentration (PAI-1:Ag), D-dimer, thrombin-antithrombin complexes, fibrinogen, activated partial thromboplastin time and international normalized ratio. The study showed significantly increased t-PA:Ag, PAI-1:Ag and D-dimer levels in patients with MPNs. Moreover, we found increased concentrations of thrombin-antithrombin complexes and fibrinogen, as well as elevated platelet counts. Detailed analysis revealed that t-PA:Ag concentration was elevated in patients with essential thrombocythemia, CML and polycythemia vera. Concentration of PAI-1:Ag was increased in patients with essential thrombocythemia and polycythemia vera; D-dimer was significantly higher in essential thrombocythemia, polycythemia vera, CML and primary myelofibrosis patients. Increased concentrations of t-PA:Ag and D-dimer indicate secondary activation of the fibrinolytic system in patients with MPNs. Elevated levels of PAI-1 in MPN patients may result from its increased production by elevated number of activated platelets and vascular endothelial damage. PAI-1 by having an inhibitory effect on fibrinolysis manifests its procoagulant activity.