Background: Serum prolidase levels change decisively in coronary artery disease and valvular disease. The aims of this study are to determine how the serum prolidase level will vary in patients who have undergone coronary bypass surgery due to mitral insufficiency and those without ischemic mitral regurgitation and those with ischemic mitral regurgitation. Methods: A total of 45 patients who were operated with coronary bypass surgery were selected. Group 1 patients were defined as patients who had coronary artery bypass surgery and did not have mitral insufficiency (25 patients), and Group 2 patients were defined as patients who had undergone coronary artery bypass surgery and had mitral insufficiency (20 patients). Before and after coronary artery bypass surgery, blood was taken from the patients and their prolidase levels were measured. Results: In Group 1 patients, pre- and postoperative serum prolidase levels were calculated as 1038.2 and 1289.43 U/L, respectively. In Group 2 patients, pre- and postoperative serum prolidase levels were calculated as 1084.07 and 1337.74 U/L, respectively. There is a significant difference between the serum prolidase (U/L) levels in the blood of group 1 and group 2 patients entering the operation and the blood after CPB (p<0.05). Conclusion: Serum prolidase level was found to be high in both patient groups. Pre- and postoperative serum prolidase levels were higher in patients with mitral valve insufficiency. Prolidase level is increased in coronary artery disease and mitral valve insufficiency.