In 1,800 patients undergoing cardiac surgery over a 2-year period, 11 incidents of abnormal inlet pressure elevations occurred before the membrane oxygenators. In 3 patients, the oxygenators had to be changed during cardiopulmonary bypass. This complication was found to be caused by fibrin formation possibly secondary to precipitation of fibrinogen with other coagulation factors in the heat exchangers of the oxygenators during the cooling phase. Large amounts of fibrin were demonstrated in the heat exchanger of the oxygenators. After careful washing of the apparatus, plasmin was added and fibrin was detected by measuring D-dimer levels. In heat exchangers from uneventful operations, only trace amounts of fibrin were found. Because there were no cold agglutinins demonstrated in the patients before surgery, cryoprecipitation studies were performed soon after surgery. When the patients' plasma samples were studied at different temperatures, from 37 degrees C down to 3 degrees C, cryoprecipitates or a gel (in 1 patient only) were formed. This indicated that there might be something abnormal with regard to fibrinogen-fibrin formation. The study patients were therefore investigated after the acute phase of the operation had ended for various coagulation factors, as well as for fibrin gel network characteristics. The results were compared with those of a control group (n = 10) with uneventful operations. There were no differences between the groups with regard to levels of coagulation factors VII and VIII and von Willebrand factor, although they were increased in both groups. The mean levels of coagulation inhibitors, antithrombin and Protein S, were slightly lower in the study patients. All of these patients had a highly pathologic, ie, tight fibrin gel network, except for the patient in whose sample a gel formed, despite being treated with aspirin or oral anticoagulants. The network was also tighter in some of the controls (v middle-aged reference individuals), although it was significantly tighter in the patients. It is concluded that some individuals who have an increased tendency to form tighter fibrin gel networks might be at increased risk for a severe complication during cardiac surgery performed under hypothermia.