Heparin-induced thrombocytopenia in patients who undergo open heart surgery. To determine whether heparin-dependent antiplatelet antibodies (HAAb) have an effect on morbidity and/or mortality rates, we reviewed the cases of 3438 patients who underwent open heart surgery from 1981 to 1989. Forty-six patients (1.3%) had HAAb. The patients were divided into two groups: those patients who were known to have HAAb before surgery (group I) and those patients who were diagnosed with HAAb after surgery (group II). Group I patients (n = 5) were pretreated with platelet-inhibiting drugs before reexposure to heparin during cardiopulmonary bypass and were maintained with strict abstinence from heparin afterward. Their lowest observed platelet counts ranged from 42,000/mm3 to 89,000/mm3 (median, 63,00/mm3). Thromboembolic complications did not occur, and all patients survived. Group II patients (n = 41) who were diagnosed to have HAAb after surgery had not been pretreated with platelet-inhibiting drugs before surgery. Lowest platelet counts ranged from 11,000/mm3 to 128,000/mm3 (median, 42,000/mm3). Bleeding complications occurred in 21 patients (51%), and thromboembolic complications occurred in 13 patients (32%). Hospital mortality in group II patients was 37%. Late recognition of HAAb was associated with an increase in morbidity and mortality rates. Thromboembolic complications of HAAb, which had been diagnosed before surgery, were eliminated, and bleeding was reduced by pretreatment with platelet-inhibiting drugs and strict abstinence from heparin after surgery. 