Thrombosis is a frequent complication of cancer, so it follows that the presence of a tumour confers a prothrombotic state. Indeed, in patients with cancer, each of the three components of Virchow's triad that predispose for thrombus formation have abnormalities, thus fulfilling the requirement for a prothrombotic or hypercoagulable state. The many signs and symptoms of the prothrombotic state in cancer range from asymptomatic basic abnormal coagulation tests to massive clinical thromboembolism, when the patient may be gravely ill. Many procoagulant factors, such as tissue factor and cancer procoagulant, are secreted by or are expressed at the cell surface of many tumours. Platelet turnover and activity are also increased. Damaged endothelium and abnormalities of blood flow in cancer also seem to play a part, as does abnormal tumour angiogenesis. Some studies have even suggested that these abnormalities may be related to long-term prognosis and treatment. We briefly describe the various clinical manifestations of thrombosis in cancer and discuss the evidence for the existence of a prothrombotic or hypercoagulable state associated with this disease. Further work is needed to examine the mechanisms leading to the prothrombotic state in cancer, the potential prognostic and treatment implications, and the possible value of quantifying indices of hypercoagulability in clinical practice.