Relative to outpatient surgery in hospital settings, ambulatory surgery centers (ASCs) are more efficient and associated with a lower cost per case. However, these facilities may also spur higher overall procedure utilization and thus lead to greater overall health care costs. The authors used the State Ambulatory Surgery Database from the State of Florida to identify Medicare-aged patients undergoing 4 common ambulatory procedures in 2006, including knee arthroscopy, cystoscopy, cataract removal, and colonoscopy. Hospital service areas (HSAs) were characterized according to ASC market share, that is, the proportion of residents undergoing outpatient surgery in these facilities. The authors then examined relationships between ASC market share and rates of each procedure. Age-adjusted rates of ambulatory surgery ranged from 190.5 cases per 1000 to 320.8 cases per 1000 in HSAs with low and high ASC market shares, respectively (P < .01). For all 4 procedures, adjusted rates of procedures were significantly higher in HSAs with the highest ASC market share. The greatest difference, both in relative and absolute terms, was observed for patients undergoing cystoscopy. In areas of high ASC market share, the age-adjusted rate of cystoscopy was nearly 3-fold higher than in areas with low ASC market share (34.5 vs 11.9 per 1000 population; P < .01). The presence of an ASC is associated with higher utilization of common outpatient procedures in the elderly. Whether ASCs are meeting unmet clinical demand or spurring overutilization is not clear.