In patients with a malignant primary disease the risk of thromboembolic complication is demonstrably increased. This is undoubtedly attributable to a whole range of factors (multifactorial etiology) which are highlighted in the present synoptic study. In addition to local influences from the tumor, a particular role appears to be played by an increased aggregation of thrombocytes, various tumor cell products, polyglobulism, hypercalcemia, increased release of thrombocinase, microthrombotic embolization, and a decrease in antithrombin III.