Background-Prior studies have demonstrated an inconsistent association between patients' arrival time for acute myocardial infarction (AMI) and their subsequent medical care and outcomes. Methods and Results-Using a contemporary national clinical registry, we examined differences in medical care and in-hospital mortality among AMI patients admitted during regular hours (weekdays 7 AM to 7 PM) versus off-hours (weekends, holidays, and 7 PM to 7 AM weeknights).