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OBJECTIVE. The purpose of this study is to investigate the magnitude of physician variation in the use of imaging and the factors associated with variation in an urban emergency department (ED) in the United States. MATERIALS AND METHODS. This retrospective cohort study was conducted from April 1, 2013, to March 31, 2014, in the ED of a level I adult trauma center in the northeastern United States. The study cohort included all patient visits to the ED during the study period. We built hierarchic and logistic regression models to determine per-physician utilization of low- and high-cost imaging, and we identified factors correlated with variation in use. Global (i.e., intraclass correlation coefficient) and individual variability metrics were used to profile physician variation after controlling for patient-, visit-, and physician-related covariates. RESULTS. A total of 56,793 patients presented to the ED during the study; of these patients, 49.5% (28,135) underwent imaging, with 38.2% (21,686) undergoing low-cost imaging and 21.9% (12,430) undergoing high-cost imaging. Statistically significant predictors of imaging orders were patient age and sex, number of secondary diagnoses, certain primary diagnoses, time of arrival in the ED, and ED crowding. Unadjusted and adjusted intraclass correlation coefficients were 0.0072 and 0.0066, respectively, for low-cost imaging, and 0.0097 and 0.0090, respectively, for high-cost imaging. The coefficient of variation for adjusted imaging odds ratios was 10.9% and 14.0% for low- and high-cost imaging, respectively, indicating a moderate degree of variation. CONCLUSION. Unexplained and moderate variation in imaging utilization exists among ED physicians, even after controlling for patient, visit, and physician characteristics. Improvement initiatives using well-defined ED imaging quality measures may help improve quality and reduce waste.