Coagulation disorders occur often in cancer patients. Thrombosis or embolism may be the first sign of an underlying malignancy. In addition, subclinical coagulation disturbances have been found in the blood of cancer patients, for example elevated concentrations of tissue factor or thrombin-antithrombin complexes. In 20% of the patients thrombocytosis occurs, and for lung and colon cancer it was found that thrombocytosis is an independent negative prognostic factor for survival. The role of an activated coagulation cascade in tumour growth is not completely clear, but there is strong evidence that the formation of a temporary fibrin matrix stimulates the formation of new blood vessels (angiogenesis) and supports tumour growth and metastasis formation. Preclinical investigations demonstrated that tumour growth and metastasis formation can be inhibited by anticoagulants. Clinical studies suggest a beneficial effect of anticoagulants on the survival of cancer patients, but phase III randomised clinical trials should be performed to determine the effect of longterm administration of anticoagulants.