Objective: The aim of the study was to evaluate the association between renin - angiotensin - aldosterone system blockers and risk of postoperative atrial fibrillation (AF) development in patients with mild and moderate left ventricular systolic dysfunction. Methods: The population of this prospective and observational study consisted of 269 patients with an ejection fraction of ≤ 50% undergoing coronary artery bypass and/or valve surgery. Use of renin -angiotensin-aldosterone system blockers (angiotensin converting enzyme inhibitors (ACEI), angiotensin receptor blockers (ARB) and spironolactone) and their association with postoperative AF (AF episode lasting > 5 min) were evaluated. In statistical analysis t test for independent samples, Chi-square test and Mann Whitney U test were used for comparison of variables between groups. Predictors of postoperative AF were determined by multiple logistic regression analysis. Results: During follow-up, 50 patients (13%) developed postoperative AF. With multiple logistic regression analysis, risk factors for postoperative AF were determined: left atrial diameter (OR- 1.09; 95%CI 1.01-1.16, p=0.02), age (OR-1.04; 95%CI 1.002- 1.08, p=0.04), aortic cross-clamp duration (OR- 1.03, 95%CI -1.00-1.05, p=0.01), use of left internal mammarian artery (OR-0.33; 95%CI 0.13-0.88, p=0.03), ACEIs treatment (OR-0.27; 95%CI 0.12-0.62, p=0.002), and ARBs treatment (OR - 0.21, 95%CI 0.07-0.62, p=0.005). Conclusions: Our results indicate that although treatments with ACEIs and ARBs are associated with low incidence of postoperative AF in patients with mild and moderate left ventricular systolic dysfunction, treatment with spironolactone is not.