Massive transfusion, or the rapid administration of a quantity of blood products that approximates an individual's blood volume, is associated with many potentially lethal complications. If the need for transfusion is immediate, ie, before adequate typing and crossmatching procedures can be completed, O negative RBCs can be given safely in the interim. Hypothermia caused by cold banked blood is aggravated by multiple environmental factors and should be aggressively avoided through the use of heat lamps, warming coils, blankets, and other warming devices. The coagulopathy seen in massive transfusion probably has a mixed etiology involving dilution and consumption of clotting factors and platelets. Although fresh frozen plasma and platelets both play a critical role in blood replacement, deficiencies should be treated with appropriate component therapy dictated by coagulation studies rather than by protocol. Transfusion reactions, the most serious type of which is the hemolytic reaction, may go unrecognized in the bleeding patient in critical condition. Hemolytic reactions can usually be prevented by careful attention to administrative and clerical accuracy. Although the overwhelming majority of the 10 million units of blood transfused annually are uncontaminated, transmission of hepatitis and the human immunodeficiency virus through blood products remains a significant screening problem. Posttransfusion hyperkalemia and acidosis are more likely to be related to inadequate resuscitation from shock than to administration of blood. Citrate toxicity and hypocalcemia are usually self-limiting disturbances. Prophylactic use of calcium chloride is dangerous and unnecessary. The complexity of the conditions necessitating massive transfusion demands frequent reevaluation of multiple laboratory and clinical factors for effective resuscitation and for safe administration of blood.