Liberation of tumor cell thromboplastins induces activation of the coagulation system and of thrombocytes with the consequence of an increased rate of thromboembolic complications in malignancies. Chemotherapy and radiation therapy there by induce also an increased rate of thromboembolic complications. Inherited thrombophilia induces an additional increase of thromboembolism in patients with malignancies and leukemias (inherited APC resistance, mutation of the prothrombin gene, protein C deficiency, protein S deficiency, antithrombin deficiency, increased lipaprotein a). Adequate anti coagulation reduces thromboembolic complications in malignancies at least partially. The increased liberation of thromboplastins by tumour cells in malignancies has according to our data another consequence: thrombin does not only act as enzyme of the haemostatic system, but also as a tissue hormone (growth factor). Thus, thrombin may induce an increased proliferation of tumour cells. Additionally, we could recently demonstrate that the preincubation of leukemic cells (HL-60) with thrombin induces resistance of these cells against the cytostatic substance idarubicin. The induction of apoptosis by idarubicin thus apparently is counteracted by thrombin. The reduction of thrombin liberation in tumor patients by anticoagulation therefore might increase the effect of idarubicin to induce apoptosis. This combined effect of anticoagulation and chemotherapy on apoptosis has to be investigated in further clinical studies.