Introduction: Coronary bypass is the most frequent cardiac surgery; individual heparin sensitivity and the anticoagulant effects of Protamine and this impact on perioperative bleeding have been reported. Objective: To establish the relationship between heparin sensitivity, Protamine: Heparin ratio and postoperative blood loss in patients undergoing coronary bypass on pump at the Fundacion Cardioinfantil. Methods: We analyze data from a adults cohort taken to coronary bypass (BCP) at Fundación cardioinfantil between january an july 2019; were classified into heparin sensitivity groups and was explored the interest variables. Results: Eighty two patients taken a coronary bypass on BCP, thirty four patients had reduced sensitivity, forty one had intermediate sensitivity and seven had heparin hypersensitivity, they were similar in terms of age, sex, body mass index, ASA, blood saving techniques, BCP time and aortic clamping time. A higher total heparin dose was used in the reduced sensitivity group, with a higher transfusion requirement during surgery. 87.8% of the patients presented a protamine: heparin ratio de less than 1:1. Postoperative bleeding with a higher transfusion requirement was evident in the intermediate sensitivity group, probably associated with a higher post-protamin final ACT. Conclusion: the protamine: Heparin ratio less than 1:1 in this cohort could explain less bleeding in the groups with reduced sensitivity and hypersensitivity, although the impression is insufficient for the group with intermediate sensitivity to heparin.