Background: Coronary artery in-stent restenosis is the negative response caused by the decrease in the artery diameters. Prolidase is an enzyme, whichplays a role in the formation of new matrix, collagen metabolism and cell development. There is no study evaluating the serum levels of prolidase enzyme in patients developing in-stent restenosis. Therefore, the objective of our study was to reveal the relationship between serum prolidase levels and in-stent restenosis. Methods: A total of 70 patients who were at medium-to-high risk based on non-invasive tests or clinical evaluation, and who underwent a coronary angiography in the cardiology and emergency polyclinics of our hospital from 2013 through 2014 were included in the study. In-stent restenosis was identified in 40 patients. In the other 30 patients, there was no angiographically determined critical lesion. Serum prolidase levels were measured in all patients. Results: The mean serum level of prolidase was found to be statistically significantly higher in the in-stent restenosis group compared to restenosis free group (p=0.02). The mean serum level of prolidase level was significantly higher in smokers compared to the non-smoker patients (p=0.04). It was observed that serum prolidase levels statistically significantly increased proportionally to the in-stent restenosis percentage (p=0.04). Conclusions: The results of this study indicate that, prolidase enzyme levels may enable timely and and correct assessment of in-stent restenosis, and may contribute to the decision for changing the treatment or timing to increase intensity of the treatment in patients undergoing percutaneous coronary intervention (PCI) with coronary stenting.