Blood loss was studied in 200 patients during and after coronary artery bypass surgery. Half of the patients were receiving dalteparin/heparin treatment for unstable angina and taking acetylsalicylic acid (ASA) 75 mg/d. One stable and four unstable patients were re-explored because of postoperative bleeding (p = 0.17). Median blood loss was 500 ml (range 100-1700 ml) during the operation and 600 ml (range 200-3400 ml) after the operation. Regression analysis showed a correlation between less postoperative bleeding and female gender and larger body mass index. In our subset of patients this correlation was not predicted from unstable angina, ASA treatment, use of aprotinin, age or cardiopulmonary bypass time. The unstable patients had lower haemoglobin levels before the operation and received blood transfusions postoperatively more frequently. A total of 101 patients were given a blood transfusion. Our findings indicate that ASA can be safely continued until the day of isolated coronary artery bypass surgery.