Atrial fibrillation (AF) is the most prevalent arrhythmia in medical practice, as well as the most frequent complication after cardiac surgery: incidence of 40%. AF has been implicated in a prolonged hospitalization, hemodynamic instability and increasing death. Considering that, in the State of Rio Grande do Norte, there’s no data collection involving post-operative AF (POAF) occurrence in cardiac surgery, an accurate investigation can contribute to prevent its occurrence and reduce its prevalence. The goal of this study is to determine the incidence of POAF after cardiac surgery in patients previously in sinus rhythm. Moreover, it’s intended to identify the clinical and laboratory profile of POAF patients, including their evolution within the hospital. A cross-sectional observational retrospective study, involving 223 patients undergoing cardiac surgery at Onofre Lopes Universitary Hospital (HUOL) from the year 2006 to 2010. The average age of the 223 patients was 58.9±14.6 years, predominantly male (61.4%). The POAF incidence was 13.9% (N=31). Coronary Artery Bypass Graft was the most commonly surgery performed (69.95%), followed by aortic valve replacement (8.52%) and mitral valve replacement (6.27%). POAF patients spent longer in intensive care unit (ICU), median of 4 days. On the other hand, non-POAF patients spent the median of 3 days (p=0.034). Mortality in POAF patients was higher (16.13%) comparing with non-POAF group (9.38%) (p=0,001). The incidence of POAF in cardiac surgery of HUOL was lower than that reported in the literature. However, it was associated with in- creased length of stay in ICU and death in the hospital course.