Non-culprit lesion severity has often been exaggerated at the time of acute ST segment elevation myocardial infarction (STEMI). We aimed to determine changes in non-culprit lesions severity on follow-up coronary angiography (CAG) and independent predictors of these changes. Material and Method: We retrospectively evaluated the changes in non-culprit lesion stenosis on follow-up CAG which was done within 2 months after primary percutaneous coronary intervention (P-PCI) in patients presenting with STEMI. Results: 154 patients were included in this study and 207 nonculprit lesions (percentage diameter stenosis (PDS) ≥50%) were compared using quantitative coronary analyses (QCA). Minimal lumen diameter (1.30±0.38 mm vs. 1.54±0.46 mm, p<0.001) and reference vessel diameter (2.88±0.66 mm vs. 2.92±0.64 mm, p=0.001) were increased significantly and PDS (54.49±9.38 vs. 47.5±11.17, p<0.001) and percentage area stenosis (78.38±8.65 vs. 71.29±11.84, p= <0.001) were decreased significantly. There was no significant change in lesion length (13.52±5.59 mm vs. 13.25±5.31 mm, p= 0.078). 65 (31.4%) of these significant lesions (PDS ≥50% by QCA) were regressed (less than 50%) on follow-up CAG. In multivariable analyses; current smoking, clopidogrel use after the P-PCI and history of coronary artery disease were the independent predictors of decrease in PDS. Conclusion: Significant exaggeration of non-culprit lesion stenosis severity occurs at the time of acute STEMI.