Background: Chest pain accounts for 8 to 10 million emergency department(ED) visits every year in the U.S., and accurately identifying patients at risk for cardiac events is critical to providing appropriate, timely, and resource-responsive care to ED patients with chest pain. Several scoring systems, including HEART score, have been developed to risk stratify patients using clinical history and objective measures in the ED. Beat to beat heart rate variability is regulated by a balance of sympathetic and parasympathetic tone. Decreased HRV has been found to be associated with all cause mortality following myocardial infarction and is also abnormal in many chronic medical conditions including hypertension, diabetes, hyperlipidemia, obesity, and smoking. Objective: The objective of our study was to test the hypothesis that HRV of patients in the ED with chest pain is associated with HEART score, and that decreased HRV would be associated with major adverse cardiac events (MACE). Methods: Prospective observational study of adults (age>30) in sinus rhythm evaluated for chest pain in a 60,000-visit academic ED between January and April, 2016. The primary clinical outcome was MACE (acute myocardial infarction, significant angiographic stenosis, or coronary intervention) during the index hospitalization or at the study hospital within 30 days. Heart rate tracings were recorded for a 10-minute period and frequency-domain measures of HRV were calculated. The correlation between HRV measures and HEART score was performed with Pearson's correlation coefficient, and the association between HRV measures and MACE was performed with univariate logistic regression. Results: Sixty patients were enrolled, of which 6 had MACE within 30 days. Most (65%) patients were admitted to the hospital. Total power and high frequency(HRV measures) were significantly associated with HEART score p=0.009, p=0.004, respectively. HRV measures were not significantly associated with MACE (all p>0.085). Conclusion: Our study showed that HRV is associated with HEART score, but likely through it's association with cardiac risk factors. Future research should focus on how HRV can be used to identify patients with cardiac chest pain very early, and specifically on predictive validity beyond risk factor identification.