Chronic myeloproliferative disorders (MPD) are characterized by a high incidence of thrombohaemorrhagic complications, possibly related to platelet abnormalities and disturbances of the coagulation system. In an attempt to define abnormalities in coagulation and fibrinolysis, we investigated risk markers for venous thromboembolism, fibrinolytic, and haemostatic system activation markers and antiphospholipid antibodies in blood samples from 50 MPD patients and 30 controls. Compared with controls median levels of protein S free and protein C were significantly decreased in the patients (0.27 vs. 0.38 arbitrary units; P < 0.001 and 0.86 vs. 0.99 arbitrary units; P < 0.001, respectively), and activated partial thromboplastin time was significantly prolonged in patients (33 vs. 27 sec; P < 0.001). No dierences were observed in levels of antithrombin, thrombin-antithrombin complex, and fibrin D-dimer. In patients the median value of thrombomodulin was significantly increased indicating perturbed endothelial function. Anticardiolipin antibodies of IgM subtype (median 38 U/mL, 33-99) were detected in 11 patients (22%) and only in one control (3%) (P < 0.021; patients vs. controls). Seven patients were heterozygous for the Factor V Leiden, one patient was heterozygous for the prothrombin G20210A mutation, and one patient was homozygous for the Factor V Leiden mutation. Among controls, two were heterozygous for the Factor V Leiden mutation. Comparing the allele frequency of the Factor V Leiden mutation in patients with MPD and the background population disclosed a significantly increased allele prevalence of the Factor V Leiden mutation in the patients (9% vs. 3.4%; P= 0.003). Alterations in the level of anticoagulant proteins, disturbances of endothelial cell function, and the presence of cardiolipin antibodies and Factor V Leiden mutation may increase the cumulative thrombotic risk in MPD besides the risk imposed by platelet activation.