The growth of ambulatory surgery centers (ASCs) as a setting for care in the United States raises unique safety concerns. In contrast to hospital-based outpatient departments (HOPDs), ASCs offer care at varying distances from hospital services potentially required to treat complications. To describe changes over time in the accessibility of hospital services for procedural outpatients, this study examined 4.3 million discharges for seven outpatient procedures frequently performed at ASCs and HOPDs. Between 2005 and 2007, the mean patient-to-emergency department (ED) distance increased by 12.4%. This change, which resulted from both an increase in the share of procedures performed at ASCs and an increase in the distance between ASCs and EDs, occurred predominantly within procedures that carried an elevated odds of hospital admission relative to the lowest risk procedures. Further research defining the risks associated with these changes in access to emergency care is needed to inform future ASC policy development and regulation.