The Swank Dacron blood filter has been clinically tested to determine if its use would improve hospital mortality following open heart surgery and the incidence of fat and nonfat emboli. The hospital mortality rate fell from 19.8% to 6.5%. Cerebral nonfat emboli were reduced from an incidence of 31% to 4.1%. Cerebral fat emboli were not affected by the filter. It is concluded that the Swank filter is an effective means of reducing nonfat microemboli during heart-lung bypass.