The pulmonary entrapment of microaggregates, platelets, and white cells in stored blood is thought to contribute to abnormalities in lung function. Washing of red cells removes these potentially damaging elements. This prospective, randomized, double-blind study compares the pulmonary function of patients undergoing abdominal aortic aneurysmectomy who received either washed or unwashed red cell concentrates. Both groups received equivalent volumes of blood, albumin, and balanced salt solution during surgery. Postoperatively, no differences were observed in the physiologic shunt, Qs/Qt, pulmonary artery pressure, pulmonary vascular resistance, or effective dynamic compliance. The cardiac index at equivalent left ventricular filling pressures was the same. The results indicate that it is unlikely that microaggregates or other formed elements in stored blood lead to compromised pulmonary function.