Two patients presented on the same day with similar stories of chest pain, physical exams, laboratory results, and ECG findings. Applying formal risk assessment provided validation of the initial impression that one patient had a higher likelihood of acute coronary syndrome than the other patient. Using pre-test probabilities of coronary artery disease, likelihood ratios of symptoms, and diagnostic tests, we demonstrate how to calculate a post-test probability for acute coronary syndrome (ACS). We discuss the importance of knowing disease prevalence and recognizing powerful tests. We also explore the variables for early risk stratification of ACS listed in the American College of Cardiology/American Heart Association guidelines. This demonstrates how the application of formal risk assessment can be used to teach clinical reasoning and improve clinical practice.