Objective To investigate the relationship between red blood cell distribution width and new onset atrial fi -brillation after acute ST segment elevation myocardial infarction in elderly patients .Methods A total of 328 patients diag-nosed with AMI in hospitalized elderly patients between January 2010 to June 2015 in Xuanwu Hospital of Capital University of Medical Sciences , department of Cardiology were enrolled .The patients were divided into AF group (42 cases) and non AF group (286 cases) by the presence or absence of AF .Clinical manifestations and echocardiography data were also evaluated in all patients.Results (1)Single factor analysis found that in the atrial fibrillation group the age , admission heart rate, RDW, high sensitive CRP (hs-CRP), NT-proBNP, left atrial diameter (LAD), LVEF, gender, smoking, the incidence of compli-cated with hypertension , killp grading of heart function , the incidence of anterior wall myocardial infarction , the use of beta blocker were significantly higher than the non-atrial fibrillation group.( t =2.096, t =3.182, t =2.026, t =3.562, t =4.082, t =2.804, t =3.092, P =0.037, P =0.002, P =0.028, P =0.001, P =0.001, P =0.012, P =0.008,χ2 =4.728,χ2 =3.165,χ2 =4.792,χ2 =14.817,χ2 =15.241,χ2 =13.469, P =0.020, P =0.038, P =0.024, P =0.001, P =0.001, P =0.001).(2)Multivariate logistic regression analysis showed that age , RDW, hs-CRP, NT-proBNP, LAD, the incidence of anterior wall myocardial infarction , the use of beta blocker were the independent predictive factors of the emerging of atrial fibrillation ( OR=1.658, OR=2.561, OR=3.841, OR=3.209, OR=4.946, OR=1.954, OR=3.815,OR=4.350,OR=2.245,95%CI 1.245-3.907,95%CI 1.641-9.720, 95%CI 1.186-13.228, 95%CI 1.209-11 .958 , 95%CI 0 .947 -20 .109 , 95%CI 1 .665-3 .842 , 95%CI 1 .167 -11 .498 , 95%CI 1 .016 -17 .817 , 95%CI 1.736-3.248).(3)Spesarman correlation analysis showed that RDW was positively correlated with inflammatory markers and NT-proBNP( r =0.31, r =0.28, P =0.001, P =0.001).Conclusion RDW is an independent risk factor for new onset atrial fibrillation after myocardial infarction in elderly patients , and its mechanism may be related to the inflammatory response and left ventricular dysfunction .