We studied the impact on triage and outcome of patients presenting to the emergency department (ED) with symptoms suggestive of an acute coronary syndromes (ACS) but without a complaint of pain. Data from a prospective clinical trial of patients with symptoms suggesting an ACS in the EDs of 10 US hospitals comparing patient demographics, clinical variables, and outcomes was used to perform a secondary analysis. Of 10,783 subjects, a final diagnosis of an ACS was confirmed in 24% of which 35% had acute myocardial infarction (AMI) and 65% unstable angina pectoris (UAP). Pain was absent in 6.2% of patients with acute ischemia and in 9.8% of those with AMI. Compared to similar patients who presented with pain, patients with painless ischemia were older, were more commonly women, had more cardiac and related diseases. Among patients with AMI, fewer patients without pain were admitted to critical care units compared to similar patients with pain. Among patients with AMI, logistic regression predicting lack of pain identified age, heart failure and diabetes, with only age and heart failure among all with ACS. After controlling for clinical features, lack of pain during acute ischemia predicted increased hospital mortality. We concluded that age and heart failure are independently associated with painless ACS, in addition to diabetes among those with AMI. Lack of pain predicts increased hospital mortality in patients with ACI through mechanisms that remain to be elucidated. There is a need for greater awareness in the general public of the different manifestations of ACS to enhance the recognition and prompt response to their symptoms. mortality