Transfusion through standard filters to dogs of stored blood containing microaggregates results in an increase in pulmonary arteriovenous shunting (Qs/Qt) and a decreased diffusion capacity of the lung for O2. These effects are due to microemboli that pass the filters and are prevented by use of Dacron wool (Swank) micropore transfusion filters. It was the purpose of this study to determine whether alterations in pulmonary shunting occur in humans following transfusions of stored blood through standard transfusion filters. In eight patients transfused over 20% of blood volumes through standard filters, Qs/Qt and alveolar-arterial O2 tension differences increased significantly. These changes did not occur in patients transfused comparable amounts of blood through Dacron wool (Swank) filters or in patients transfused less than 20% of blood volumes. A direct correlation was found between the absolute percent change in Qs/Qt and the quantity of microaggregates passing the filter and present in the transfused blood. It is concluded that removal from stored blood of microaggregates by administration of the blood through effective micropore transfusion filters prevents an increase in Qs/Qt caused by administration of such material.