Bleeding can be considered unexpected if it is disproportionate to the intensity of the haemostatic stress in a patient with no known haemorrhagic disorder or if it occurs in a patient in whom a bleeding disorder has been characterized but is adequately treated. A thorough history usually allows the clinician to predict reasonably accurately whether the patient is likely to have a systemic haemostatic defect (and if so whether it is congenital or acquired), or whether the bleeding likely has a purely anatomical basis. The nature of bleeding is instructive with respect to preliminary categorization. Thus, mucocutaneous bleeding suggests defects of primary haemostasis (disordered platelet-vascular interactions). Bleeding into deeper structures is more suggestive of coagulation defects leading to impaired fibrin clot formation, and delayed bleeding after primary haemostasis is characteristic of hyperfibrinolysis. Localized bleeding suggests an anatomical cause, although an underlying haemostatic defect may coexist. Where bleeding is so acutely threatening as to require urgent intervention, diagnosis and treatment must proceed simultaneously. In the case of minor haemorrhage (not threatening to life or limb) it may be preferable to defer therapy while the nature of the bleeding disorder is methodically investigated. Initial laboratory evaluation is guided by the preliminary clinical impression. The amount of blood loss can be inferred from the haematocrit or haemoglobin concentration, and the platelet count will quickly identify cases in which thrombocytopenia is the likely cause of bleeding. In the latter instance, examination of the red cell morphology, leucocyte differential, and mean platelet volume may allow the aetiological mechanism to be presumptively identified as hypoproliferative or consumptive. With regard to coagulation testing, the activated PTT, prothrombin time, and thrombin time usually constitute an adequate battery of screening tests, unless the clinical picture is sufficiently distinctive to indicate the immediate need for more focused testing. In any event, sufficient blood should be taken to allow more detailed studies to be done based on the results of these screening tests. These results will direct the need for further assays, such as specific clotting factor activity levels, von Willebrand factor assays, tests for coagulation inhibitors, platelet function assays, and markers of primary or secondary fibrinolytic activity.