Objective: To determine the incidence of postoperative atrial fibrillation (PAF) of cardiac surgery, its impact on morbimortality and duration of hospital stay in a tertiary cardiology center of the state of Santa Catarina, Brazil. Methods: Cohort study with 134 adult patients submitted to cardiac surgery. Results: the incidence was 32.8%. After multivariate analysis, patients who did not receive beta-blockers were associated with PAF with a relative risk odds ratio (RR) 10.73 (p <0.001). The highest rate of cardiovascular events (cerebrovascular accident, mortality, and acute coronary syndrome) was 25% in the PAF group. 10% (RR 3.21; p = 0.035) which, consequently, generated longer hospitalization time in these patients (19.1 vs. 12.5; p = 0.01). Conclusion: the incidence of PAF was high, caused a significant increase in morbimortality and duration of hospital stay, and consolidated the role of beta-blocker therapy in its prevention, and may serve as a basis for future prevention policies.