Many anomalies in primary haemostasis, coagulation and fibrinolysis have been described which favour thrombosis in cancer patients. Cancer is often associated with other risk factors for thromboembolism which explain much of the increased frequency of thrombosis in these patients. Thromboembolism may precede cancer and sometimes is the inaugural manifestation leading to the diagnosis of cancer. Cancer occurs more frequently in patients with idiopathic recurrent venous thrombosis than in those with thrombosis due to other risk factors for thromboembolism. The results of antivitamin K therapy are disappointing: in retrospective series, recurrent thromboembolism occurred in 9% of the treated patients and severe haemorrhage in 27%. New therapies should be assessed in cancer patients with thromboembolism.