['To evaluate whether patients with positive or negative heparin antibodies who received heparin preoperatively by continuous infusion developed cardiovascular changes upon heparin administration prior to cardiopulmonary bypass.', 'Clinical trial.', 'Single institution, academic hospital.', 'Eighty (80) patients with good ventricular function on low-dose heparin infusion prior to surgery.', 'Patients were divided into 2 equal groups: group A had negative heparin antibodies (% ratio < 0.26), group B had positive heparin antibodies (% ratio > 1.2). All patients received heparin, 400 units/kg, prior to institution of cardiopulmonary bypass. Cardiovascular changes, activated coagulation time (ACT), and histamine levels were measured before and 5 minutes after administration of heparin. Platelets also were counted before and 6 hours after surgery.', 'Significant hypotension and decreased cardiac index occurred in patients with positive heparin antibodies who received heparin prior to cardiac surgery. Histamine levels increased significantly 5 minutes after heparin administration. Significant thrombocytopenia occurred 6 hours after surgery in group B patients. There was a good correlation between heparin antibodies, histamine levels, thrombocytopenia and cardiovascular changes. Group B patients also had heparin resistance as manifested by a lower ACT after the loading doses of heparin.'] Does preoperative heparin therapy cause immune-mediated hypotension upon heparin administration for cardiac surgery?