Most patients admitted to an intensive care unit (ICU) require the administration of one or more blood components during their stay. Such patients exhibit great diversity in conditions necessitating care in the ICU, age, underlying medical problems, and integrity of physiologic compensatory mechanisms. All these patients, however, share the need for optimized oxygen-carrying capacity and tissue perfusion. Ongoing blood loss resulting from injuries, surgical wounds, invasive monitoring equipment, and blood sampling requirements, coupled with inadequate marrow function and, in some, red cell destruction,