Red cell deformability was observed during open-heart surgery in 59 patients. Deformability, assessed with a standard microfiltration method, was expressed as red cell filtration rate (RFR) in μl/s. The mean preoperative value, 38.9±1.0 μl/s, showed a generally falling tendency. The first significant decrease in mean RFR (by 18%) followed induction of general anaesthesia, and the second (by 15 %) was seen 60 min after the start of cardiopulmonary bypass (CPB). The major decrease in RFR during CPB was found at the end of bypass, when it was reduced to 62% of the pre-CPB value. The percentage RFR reduction at the end of CPB showed significant correlation with 1) CPB duration (r=0.49), 2) oxygen flow rate index (OFRI), i.e. flow/min in the bubble oxygenator/m² bsa (r=0.38), and 3) blood flow rate index (BFRI), i.e. average volume of blood pumped through the heart-lung machine/min CPB time/m² bsa (r=0.51).