The routine preoperative performance of a platelet count, bleeding time, prothrombin time, and partial thromboplastin time will reveal most of the major inherited or acquired bleeding diatheses that are likely to be troublesome during or after an operation. Unfortunately, these screening test may fail to warn the physician of impending trouble because they fail to detect the milder bleeding tendencies. In such cases asking the patient a few specific questions may reveal a bleeding tendency not otherwise suspected. If a patient with an identified bleeding problem undergoes a surgical procedure, operative and postoperative bleeding can be minimized by continuous monitoring of the patient by the coagulation laboratory and by close collaboration among the surgeon, the hematologist, and the laboratory in the management of replacement therapy. However, if bleeding commences during or after an operation without warning, the nature of the problem must be recognized as quickly as possible in order to effect prompt treatment. By a series of simple tests, the coagulation laboratory can promptly identify the general nature of the bleeding problem and then more leisurely pinpoint it precisely.