We propose to perform a prospective, randomized, double-blind clinical trial in critically ill patients whose lungs are mechanically ventilated, and who have orders to receive transfusion of at least one unit of packed red cells, comparing pulmonary function between patients randomly allocated to receive a unit of red cells stored for 5 days or less, with similar patients randomly allocated to receive a unit of red cells stored conventionally (median 21 days). We will test the hypothesis that transfusion of red cells of lesser storage age, (5 days or less) will produce the less changes of pulmonary function and immune activation than will transfusion of red cells of conventional storage age (median 21 days).