Background Although atrial fibrillation is common in critically ill patients, no large studies on its impact on patient mortality in general intensive care units have been done. Objective To evaluate the association between atrial fibrillation and hospital mortality in critically ill patients. Methods In a retrospective cohort study, critically ill patients who had atrial fibrillation during a 2-year period were compared with patients who did not. The primary outcome was death during the hospital stay. Secondary outcomes were duration of mechanical ventilation and lengths of stay in the intensive care unit and hospital. Results Among a total of 2018 first-time admissions to the intensive care unit during the study period, 421 patients (20.9 ) had atrial fibrillation. Patients with atrial fibrillation had higher mortality, significantly longer duration of mechanical ventilation, and longer stays in the intensive care unit and in the hospital than did patients without this cardiac arrhythmia. However, multiple logistic regression analysis indicated that atrial fibrillation was not independently associated with a higher risk for death. Conclusion Atrial fibrillation may not be independently associated with hospital mortality.