To assess agreement between emergency physicians' measurements of abdominal aortic diameter using ultrasound in the Emergency Department (ED) and measurements obtained by computed tomography (CT), a double-blinded, prospective study was conducted. The study enrolled a convenience sample of patients over 50 years of age presenting to the ED and scheduled to undergo CT scan of the abdomen and pelvis. Before CT scan, each patient received an ultrasound from a resident or attending emergency physician measuring anterior-posterior aortic diameter transversely at the approximate level of the superior mesenteric artery (SMA), longitudinally midway between the SMA and the iliac bifurcation, and transversely approximately 1 cm above the iliac bifurcation. Two radiologists blinded to the ultrasound measurements then independently measured aortic diameters at the corresponding anatomical points as imaged by CT. The ultrasonographic measurements were then compared with an average of the two CT measurements. Forty physicians enrolled a total of 104 patients into the study. Ultrasonographic measurements of aortic diameter were slightly smaller than those obtained by CT scan, with a difference of means of -0.39 cm (95% CI -0.25 to -0.53) at the level of the SMA, -0.26 cm (95% CI -0.17 to -0.36) on longitudinal view, and -0.11 cm (95 % CI -0.01 to 0.22) at the bifurcation. At the level of the SMA, the difference in measurements by ultrasound and CT would be expected to be less than 1.41 cm, 95% of the time. At the bifurcation, we expect 95% of the differences to be less than 1.05 cm. Agreement was closest on longitudinal view, with 95% of the differences expected to be less than 0.94 cm. Participating physicians estimated the time required to complete their ultrasound studies to be less than 5 min in a majority of cases. In conclusion, ultrasonographic measurement of aortic diameter by emergency physicians rapidly and effectively approximates measurements obtained by CT scan.