De novo atrial fibrillation (AF) is common in the acute coronary syndrome (ACS), but most patients (P) are discharged in sinus rhythm (SR). The recurrence of arrhythmia in follow-up (FU) remains unknown and its undervaluation may have prognostic impact.

To characterize a population with ACS and de novo AF and determine its prognostic impact.

To evaluate the recurrence of AF and the incidence of ischemic stroke/systemic embolism in FU.

We analyzed 2383 P consecutivelly admitted with ACS and with a minimum of 180 days FU.

De novo AF has been defined as AF first detected on admission or during hospitalization.

It has been observed de novo AF in 199 P (8.4%), the majority being male (70.4%). These patients were significantly older (p<0.001) and had higher HTA prevalence (p<0.001.

The echocardiographic evaluation showed a higher prevalence of mitral insufficiency (p<0.001) and left ventricular ejection fraction<40% (p<0.001). These P developed more frequently heart failure (p<0.001) and stroke (p=0.001) during hospitalization.

The in-hospital mortality (9.5% vs 3.3%, p<0.001) and at 6 months (15.6% vs 6.6%, p<0.001) were also higher, however, the presence of de novo AF was not an independent predictor of these events. Most P were discharged in SR (n=134) and without anticoagulant therapy (n=117).

95 patients with de novo AF were followed for a median period of 690 days and it was observed the occurrence of ischemic stroke/systemic embolism in 9 P (9.5%), but only one was under oral anticoagulation.

AF occurred in 37.5% of P that were in SR on discharge.

The presence of de novo AF was associated with increased risk of adverse events, although it may constitute only a marker of severity.

We observed recurrence of AF in a considerable sample, which denotes the need for appropriate evaluation of thromboembolic risk of these P.