The purpose of this study is to determine the incidence, risk factors, and outcomes after the development of an atrial arrhythmia (AA) in trauma patients admitted to the intensive care unit (ICU). We performed a retrospective study of more than 7 years of trauma patients admitted to the ICU at an urban, academic Level I trauma center. Patients with AA, defined as atrial fibrillation, atrial flutter, or paroxysmal supraventricular tachycardia, were compared with patients without AA. Groups were compared by univariate and multivariate analysis. Three thousand, four hundred and ninety-nine trauma patients were admitted to the ICU during the study period and 210 (6%) developed an AA. AA patients were more likely to sustain blunt trauma, were older, more often female, more severely injured, and sustained more head injuries. The only independent risk factor for developing an AA was age > 55 years (odds ratio = 4.6, P < 0.01). Mortality was higher in the AA group (33% vs. 14%, P < 0.01) and AA was an independent risk factor for mortality (odds ratio = 1.7, P = 0.01). Twenty-eight per cent (n = 59) of AA patients received beta-blockers in the postinjury period, and these patients had lower mortality (22% vs. 37%, P = 0.04). AA occurs in 6 per cent of trauma patients admitted to the ICU. Developing an AA is an independent risk factor for mortality after trauma. Beta-blocker therapy was associated with decreased mortality in trauma patients with AA.