Aim:To evaluate the clinical impact of reduced heparin responsiveness (HR(reduced)) on the incidence of myocardial infarction (MI) following off-pump coronary artery bypass graft surgery (OPCAB), and to identify the predictors of HR(reduced).Methods:A total of 199 patients scheduled for elective OPCAB were prospectively enrolled. During anastomosis, 150 U/kg of heparin was injected to achieve an activated clotting time (ACT) of ≥ 300 s, and the heparin sensitivity index (HSI) was calculated. HSIs below 1.0 were considered reduced (HR(reduced)). The relationships between the HSI and postoperative MI, cardiac enzyme levels and preoperative risk factors of HR(reduced) were investigated.Results:There was no significant relationship between the HSI and cardiac enzyme levels after OPCAB. The incidence of MI after OPCAB was not higher in HR(reduced) patients. HR(reduced) occurred more frequently in patients with low plasma albumin concentrations and high platelet counts.Conclusion:HR(reduced) was not associated with adverse ischemic outcomes during the perioperative period in OPCAB patients, which seemed to be attributable to a tight prospective protocol for obtaining a target ACT regardless of the presence of HR(reduced).