OBJECTIVE: Hypercoagulability is a commonly described complication in patients with Cushing's syndrome. Recent clinical studies have indicated various abnormalities of coagulation and fibrinolysis parameters which may be related to that phenomenon. The aim of this study was to investigate the mechanisms underlying the hypercoagulable state in patients with Cushing's syndrome. ----- RESEARCH METHODS AND PROCEDURES: A wide range of serum markers involved in the processes of blood coagulation and fibrinolysis was measured in a group of 33 patients with Cushing's syndrome and 31 healthy controls. No participant was taking medication which could influence the result or had known diseases, except hypertension and diabetes, which could affect blood coagulation or fibrinolysis parameters. ----- RESULTS: Patients with Cushing's syndrome had higher levels of clotting factors II (P = 0.003), V (P < 0.001), VIII (P < 0.001), IX (P < 0.001), XI (P < 0.001) and XII (P = 0.019), protein C (P < 0.001), protein S (P < 0.001), C1-inhibitor (P < 0.001) and plasminogen activator inhibitor-1 (PAI-1) (P = 0.004). The activity of fibrinolytic markers, plasminogen (P < 0.001), antithrombin (P < 0.001) and antithrombin antigen (P = 0.001) was also increased in the patient group. ----- CONCLUSION: The study has demonstrated hypercoagulability in patients with Cushing's syndrome manifest as increased prothrombotic activity and compensatory activation of the fibrinolytic system. We propose the introduction of thromboprophylaxis in the preoperative and early postoperative periods, combined with a close follow-up in order to prevent possible thromboembolic events in patients with Cushing's syndrome.