- There is scarcity of data on atrial fibrillation (AF) prevalence and outcomes in critically ill patients admitted to the non-cardiac Intensive Care Unit (ICU). The purpose of our study was to prospectively assess the incidence, risk factors and prognosis of new onset atrial fibrillation in a medical non-cardiac ICU population. A prospective single center observational study was conducted in an 8-bed adult Medical Intensive Care Unit (MICU). The patients at the MICU were continuously monitored throughout their stay, once an irregularly irregular rhythm was recorded, a confirmative 12-leads ECG was performed and the incidence, duration of the atrial fibrillation and clinical signs of the patient were logged into the computerized patient file. A total of 209 patients were included in the study; 23 of these patients developed AF during the hospital stay. Out of the 209 patients, 26% died during the period of hospitalization. There were a few significant differences between the AF group and the non-AF group including age (67 vs. 49, p < 0.001), proportion of Arab Bedouins (4% vs. 28%, p = 0.01), prevalence of coronary heart disease (39% vs. 10%, p < 0.001), paroxysmal atrial fibrillation (52% vs. 3%, p < 0.001), hypertension (70% vs. 31%, p < 0.001 and dyslipidemia 166% vs. 24%, p < 0.001). The APACHE II scores were similar in both groups. In the AF group, there were more patients with sepsis at admission compared with the non-AF group. Age, length of stay, paroxysmal atrial fibrillation (PAF) and dyslipidemia were independent factors for the AF development. AF occurrence adjusted for APACHE-II score was not a significant predictor of death during hospitalization (OR = 1.55, p = 0.38). The development of AF is more prevalent in patients with PAF. Development of AF was not found to be an independent mortality risk factor.