Patients with ST-segment elevation myocardial infarction (STEMI) admitted during nonregular working hours (off-hours) have been reported to have greater mortality than those admitted during regular working hours (on-hours), perhaps because of the lower availability of catheterization laboratory services and longer door-to-balloon times. This might not be the case, however, for hospital centers in which primary percutaneous coronary intervention (PCI) is invariably performed. We conducted a substudy using the Harmonizing Outcomes with Revascularization and Stents in Acute Myocardial Infarction study data to determine whether the STEMI arrival time was associated with differing clinical outcomes. We identified all patients with STEMI admitted to a PCI-capable hospital who underwent primary PCI. Patients presenting during on-hours were compared to those presenting during off-hours. The primary outcome of death, major adverse cardiovascular events, and net adverse clinical events was examined. We identified 2,440 patients (1,205 [49%] on-hours and 1,235 [51%] off-hours). Similar baseline characteristics were observed. The off-hour patients had a significantly longer door-to-balloon time (92 vs 75 minutes; p <0.0001) and total ischemic time (209 vs 194 minutes; p <0.0001). Despite these differences, the risk-adjusted all-cause mortality, major adverse cardiovascular events, and net adverse clinical events rates were similar for both groups during the in-hospital, 1-year, and 3-year follow-up. In conclusion, patients with STEMI presenting to primary PCI hospitals during off-hours might have slightly longer delays to revascularization; however, they experienced similar short- and long-term survival and clinical outcomes as those arriving during on-hours.