Introduction: Aortic dissection is a rare event. While the most frequent symptom is chest pain, that is a common emergency department (ED) chief complaint and other diseases causing chest pain occur much more often. Furthermore, 20% of dissections are without chest pain and 6% are painless. For these reasons, diagnosing dissections may be challenging. Our goal was to determine the number of total ED and atraumatic chest pain patients for every aortic dissection diagnosed by emergency physicians.Methods: Design: Retrospective cohort. Setting: 33 suburban and urban New York and New Jersey EDs with annual visits between 8,000 and 80,000. Participants: Consecutive patients seen by emergency physicians from 1-1-1996 through 12-31-2010. Observations: We identified aortic dissection and atraumatic chest pain patients using the International Classification of Diseases 9th Revision and Clinical Modification codes. We then calculated the number of total ED and atraumatic chest pain patients for every aortic dissection, along with 95% confidence intervals (CIs).Results: From a database of 9.5 million ED visits, we identified 782 aortic dissections or one for every 12,200 (95% CI [11,400-13,100]) visits. The mean age of dissection patients was 66±16 years and 38% were female. There were 763,000 (8%) with atraumatic chest pain diagnoses. Thus, there is one dissection for every 980 (95% CI [910-1,050]) atraumatic chest pain patients.Conclusion: The diagnosis of aortic dissections by emergency physicians is rare and challenging. An emergency physician seeing 3,000 to 4,000 patients a year would diagnose an aortic dissection approximately every three to four years.