Febrile and allergic reactions are the most common adverse events associated with the transfusion of blood components. Most febrile reactions are caused by biological response modifiers that accumulate in the component during storage or are the result of leucocyte antigenantibody reactions. Leucocyte reduction of blood components has decreased the frequency of these reactions; however, when they do occur the challenge for the clinician is to rule out other potential causes of the reaction that may be associated with severe morbidity or mortality. Allergic reactions are most commonly due to reactions against plasma proteins. Symptomatic relief of mild urticarial reactions can usually be achieved with the use of antihistamines, but premedication does not appear to reduce the incidence of allergic reactions. Although anaphylactic reactions are uncommon, close monitoring of patients during transfusions permits prompt termination of the transfusion and treatment to minimize further morbidity or mortality.