During the past 20 years, the perceived value of blood transfusions has changed as it has become appreciated that transfusions are not without risk. Red blood cell transfusion has been associated with disease transmission and immunosuppression for some time. More recently, proinflammatory consequences of red blood cell transfusion have also been documented. Moreover, it has become increasingly evident that stored red blood cells undergo time-dependent metabolic, biochemical, and molecular changes. This 'storage lesion' may be responsible for many of the adverse effects of red blood cell transfusion. Clinically, the age of blood has been associated with multiple organ failure, postoperative pneumonia, and wound infection. The relationship between age of blood and clinical adverse effects needs further study.