Active blood warming is a recent practice and arises out of conflicting needs. On the one hand, the safety and preservation of blood require refrigerated storage and delivery up to the moment of transfusion. On the other hand, modern methods of very rapid transfusion in resuscitation would cause clinically dangerous hypothermia if unmodified, ice-cold blood were to be so transfused. These needs must be reconciled in the interest of adequate patient care--hence the need for blood warming. Nevertheless, blood warming creates risks of its own and should not be used without justifying clinical indications. Within limits that extend somewhat above normal body temperature, the application of heat does no harm to stored RBC, a fact that is not reflected in current standards for blood warmers. Bearing in mind the human tendency to "stretch" standards and the fallibility of mechanical devices, caution is always wise. But perhaps the time has come for reconsideration of the present upper limit of 38 degrees C. Many varieties of blood warmers are available in the US, but none at this time is based on electromagnetic activity. The most common systems now in use are in-line warmers, most of which are not adequate for the type of rapid-transfusion systems currently available. Countercurrent in-line blood warmers and the method of rapid warm saline admixture can both be used successfully for rapid, massive transfusions. Blood warming is seldom necessary or desirable for elective transfusions at conventional rates, even for patients with cold autoagglutinins.