Background. Heparin-induced thrombocytopenia (HIT) is a potentially devastating complication of heparin therapy. The incidence of clinical HIT after cardiovascular surgery is less than 2%, although asymptomatic antibodies to heparin-platelet factor 4 (PF4) occur more frequently. Bovine heparin is thought to cause more HIT than porcine heparin, although this has never been established for heparin use during coronary,artery bypass grafting. We therefore undertook a randomized, prospective study of heparin-PF4 antibody formation in patients undergoing first-time CABG given intraoperative bovine or porcine heparin. Methods. Two hundred seven patients (108 porcine, 99 bovine) completed the study. Heparin given pre- or postoperatively was always porcine. Platelet counts and heparin-PF4 antibody tests (enzyme-linked immunosorbent assays) were performed preoperatively and daily until postoperative day 7 or discharge if earlier. Results. The overall incidence of heparin-PF4 antibody formation was 42%. Six patients (2.9%) were positive preoperatively, of which, 1 developed clinical HIT. When these were excluded, seroconversion rates were 44 of 99 (44.4%) and 33 of 108 (30.6%) for bovine and porcine heparin, respectively (p = 0.041). Among patients who produced antibodies, most (90% bovine, 85% porcine) seroconverted after postoperative day 2. There were no differences in postoperative platelet counts; only 1 patient developed thrombosis associated with seroconversion, but without developing thrombocytopenia. The seroconversion rates for patients having cardiopulmonary bypass or off-pump surgery were not significantly different. Conclusions. This study confirms the high frequency of heparin-PF4 antibodies after coronary artery bypass grafting and demonstrates a significantly higher incidence after bovine heparin. However, because some patients may seroconvert after discharge, our study may underestimate the true incidence. (C) 2003 by The Society of Thoracic Surgeons.