Objectives: the purpose of this study was to examine the impact of coronary artery bypass surgery without the use of CPB on post-operative blood loss, transfusion requirements, and platelet functions. Background: One of the advantages claimed with the use of OPCAB is the avoidance of the generalized hemostatic deficit, which was long attributed to the use of CPB. OPCAB also offers an excellent chance to examine the impact of the surgery and the relatively high dose of heparin on platelet function after coronary bypass surgery. Methods: sixty patients with coronary artery disease were prospectively randomized to (1) on-pump treatment with conventional cardiopulmonary bypass and (2) off-pump treatment on the beating heart. Patients were examined for post-operative blood loss & transfusion requirement, and need for re-exploration. Platelet studies were undertaken at two hours post-operatively and included platelet count, mean platelet volume, ADP platelet aggregation, and flow cytometric analysis for P-selectin & annexin V positive platelets, which were used as markers of platelet activation.Results: there was no statistically significant difference between both groups regarding post-operative blood loss or need for re-exploration for bleeding. More patients of the conventional CABG group required transfusion, but the difference did not attain statistical significance. However, the number of units of blood constituents used in the on-pump group was statistically higher than that used in the off-pump group. Platelet studies revealed definite platelet dysfunction in the on-pump group, however there was still statistically significant elevation of markers of platelet activation in the off-pump group.Conclusion: post-operative bleeding is similar whether or not CPB was used but at the expense of more blood constituents transfusion in the on-pump group. Platelet activation still occurs even when CPB is avoided. This may correlate with previous reports of pro-coagulant states of OPCAB.