Thirty-four patients were entered into a non-blinded, randomized study to test the effect of preoperative aspirin ingestion on postoperative blood loss and transfusion requirements after coronary artery bypass grafting. Sixteen patients in the aspirin-treated group had significantly increased chest-tube blood loss 12 hours after operation (1,513 ± 978 versus 916 ± 482 ml; p = 0.038). In addition, aspirin users had significantly increased requirements for postoperative packed red blood cells (4.4 ± 3.5 versus 1.8 ± 1.3 units; p = 0.014), platelets (1.3 ± 1.3 versus 0.2 ± 0.4 six-donor units, p = 0.0049), and fresh-frozen plasma (3.6 ± 5.0 versus 0.78 ± 1.6 units; p = 0.042) transfusions. The only patients requiring reoperation for bleeding were in the aspirin-treated group (2 patients). Six patients were not entered into the randomized part of the study because of excessively prolonged post-aspirin bleeding times (〉 10 minutes). This finding suggests that a subset of patients are particularly sensitive to aspirin and have significantly prolonged bleeding times after aspirin ingestion. We conclude that aspirin ingestion increases postoperative blood loss and transfusion requirements, and we recommend discontinuation of aspirin therapy before cardiac procedures.