Severe hemostatic defects that occur during massive transfusion are related to the volume of blood transfused, preexisting hemostatic abnormalities, concomitant pathologic changes, and therapeutic maneuvers. The relative role of each factor in the bleeding can be rapidly determined by using routine clinical laboratory tests. This determination requires an understanding of the properties of selected clotting factors, what coagulation screening tests measure, how these tests behave as the levels of factors change, and test profiles characteristic of different defects. Screening tests include platelet count, prothrombin time, partial thromboplastin time, thrombin time, and fibrinogen level. These tests are generally available on an emergent basis and can be completed within 25 minutes. The pattern associated with hemodilution is universal in massive transfusion. Patterns characteristic of the other pathologic processes that may be encountered are simply superimposed on the characteristics of hemodilution. Successful management of the contributing causes of bleeding depends on the administration of the appropriate blood components in the dose necessary to ensure that the levels of platelets and clotting factors are returned to and maintained at hemostatic levels.