MR-pro ANP is a novel peptide with multiple biological functions, especially within the cardiovascular system. It plays an important role in physiological and pathological processes in the human body and its level may be clinically relevant in new onset atrial fibrillation. The aim of this study was to determine whether elevated MR-proANP level is a predictor of new-onset atrial fibrillation in patients with acute myocardial infarction. The study included 96 patients with their first diagnosis of acute myocardial infarction hospitalized in the Department of Interventional Cardiology within one year. The plasma levels of MR-proANP were determined on admission, and on the first and fifth day of hospitalization by BRAHMS MRproANP KRYPTOR. New onset AF was diagnosed in 9.4% of patients during hospitalization; the level of MR proANP was also found to be significantly higher in this group of patients compared to those without arrhythmia (p = 0.017). ROC curve analysis indicates that the concentration of MR-proANP is a good predictor of AF diagnosed during the hospitalization in patients with acute myocardial infarction (AUC =0.738, p = 0.0011). MR-proANP might be a good predictor of new-onset atrial fibrillation in patients with acute myocardial infarction.