Patients with cancer have an increased incidence of thromboembolic disease and haemostatic abnormalities, and there is considerable evidence that the haemostatic system is involved in the growth and spread of malignant disease. Anti-haemostatic agents have given promising results in the treatment of experimental tumours, and several clinical trials in humans have been initiated. The formation of fibrin around the tumour may be a particularly important factor in malignant dissemination. The precise mechanisms of peri-tumour fibrin deposition remain to be elucidated, but may involve alterations in local vascular permeability and the presence of tumour and/or macrophage procoagulants. In addition to their role in fibrin formation, haemostatic components may also be involved in neovascularisation and angiogenesis.