Hemostatic abnormalities associated with malignancy have been described since the middle of the 19th century. Abnormalities associated with hypercoagulability and hemorrhage are reported in various percentages of patients depending upon the underlying neoplasm and the type of therapy. Changes in the quantitative and qualitative aspects of protein coagulation factors, anticoagulant proteins, circulating anticoagulants, platelets, and vascular responses have been noted. Clinical or subclinical disseminated intravascular coagulopathy (DIC) and associated paradoxical bleeding are common. Hemorrhage may be associated with a decrease of particular coagulation factors or alterations of vascular integrity and platelet numbers or function in various combinations. Evaluation of hemostatic abnormalities associated with cancer (HAAC) includes a careful history and physical examination, assessment of the prothrombin and activated partial thromboplastin times, platelet count, a test for fibrin or fibrinogen degradation products, and assay of fibrinogen levels. Specific findings may suggest the need for tests for naturally occurring protein anticoagulants (e.g., protein S, protein C, and antithrombin III), coagulation inhibitors, abnormalities of the fibrinolytic system, or other esoteric tests. Testing for F1 + 2 and fibrinopeptide A may be useful in determining early activation of prothrombin and thrombin, respectively, and a clue to incipient onset of DIC. Besides the disease, therapies for cancer can alter hemostatic activity. Chemotherapy has been reported to be associated with venous and arterial thromboses, cerebrovascular events, and coagulopathies. Radiation therapy decreases platelet production, particularly if the active bone marrow has been included in the field. Laboratory evaluation of HAAC requires consideration of the type of malignant disorder, the history and physical condition of the patient and any therapy.