In 56 patients undergoing open-heart surgery, red cell trauma during and following cardiopulmonary bypass (CPB) was monitored with a microfiltration method that estimated deformability of the cells. Red cell deformability was reduced by 38% during CPB and at a slower rate thereafter. The lowest filterability rate was reached on the second day, after which improvement began. In patients who had undergone coronary artery bypass grafting, preoperative values of red cell deformability were reached after 6 weeks. Following valve replacement, however, preoperative values were not regained during this period, which was attributed to continuous mechanical trauma by the artificial valves. A 50% reduction of red cell filterability from the end of CPB to 12 and 24 hours from the start of CPB was associated with heavy blood loss (> 1000 ml) from drains. Reduced red cell deformability thus showed relationship with a bleeding tendency following use of CPB.