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Introduction: Oral beta-blockers improve the prognosis of patients with acute myocardial infarction, while atrial fibrillation worsens the prognosis of this population. The reduction of atrial fibrillation incidence in patients treated with beta-blockers could at least in part explain the benefits of this drug., Objective: To investigate the effect of beta-blockers on the incidence of atrial fibrillation in patients with acute myocardial infarction., Methods: We analyzed 1401 patients with acute myocardial infarction and evaluated the occurrence or absence of atrial fibrillation, the use of oral beta-blockers and mortality during the first 24 hours., Results: a) The use of beta-blockers was inversely correlated with the presence of atrial fibrillation (rho = 0.004; OR = 0.54). b) Correlations with mortality were as follows: 31.5% in patients with atrial fibrillation, 9.2% in those without atrial fibrillation (rho < 0.001; Odds Ratio = 4.52), and 17.5% in patients not treated with beta-blockers and 6.7% in those who received the drug (rho < 0.001; OR = 0.34). c) Adjusted Models: The presence of atrial fibrillation was independently correlated with mortality (OR = 2.48, rho = 0.002). The use of beta-blockers was inversely and independently correlated with mortality (OR = 0.53; rho = 0.002). The patients who used beta-blockers showed a lower risk of atrial fibrillation (OR = 0.59; rho = 0.029) in the adjusted model., Conclusion: The presence of atrial fibrillation and the absence of oral beta-blockers increased in-hospital mortality in patients with acute myocardial infarction. Oral beta-blockers reduced the incidence of atrial fibrillation, which might be at least partially responsible for the drug's benefit.