A large number of coronary patients referred for coronary bypass surgery receive platelet anti-aggregant therapy which has the disadvantage of increasing per and postoperative haemorrhage. The aim of this study was to assess the effects of aprotinin in 60 patients under platelet antiaggregant therapy (aspirin 250 mg/day) for over 30 days before surgery for coronary bypass grafting. The clinical (age, weight, diagnosis, bypass time, number of grafts) and biological features (preoperative haemoglobin concentration, platelet count and fibrinogen levels) were identical in a group treated by aprotinin (Group A, n = 30) and a control group (Group B, n = 30). The aprotinin treatment protocol was an intravenous injection of 2 million KIU (kallikrein inhibitory units) before starting the cardiopulmonary bypass and 2 million KIU in the filling liquid of the bypass circuit. A significant reduction in blood loss was observed in Group A with respect to Group B (370 +/- 154 ml versus 651 +/- 323 ml at day 1, p < 0.01). The haemoglobin concentration was higher in Group A than in Group B (11.9 +/- 1.6 g/100 ml versus 10.3 +/- 1.4 g/100 ml, p < 0.001) and fewer patients required blood transfusion (Group A: 16% versus Group B: 43%, p = 0.04). In conclusion, high doses of aprotinin given to patients on aspirin therapy undergoing coronary bypass surgery, significantly reduce postoperative blood loss and the number of patients transfused.