Ambulatory surgery centers (ASCs) play an important role in providing surgical and diagnostic services in an outpatient setting. They can be owned by physicians who staff them. Previous studies focused on patient “cherry picking” and over-utilization of services due to physician ownership. Few studies examined the relationship between physician ownership and quality of care. Using a retrospective cohort of patients who underwent colonoscopy, this study examined the effect of physician ownership of ASCs on the occurrence of adverse events after outpatient colonoscopy. Agency theory is used to as a conceptual framework. Depending on the extent to which consumers are able to assess quality of care differences across health care settings, physician ownership can function as a mechanism to improve quality or as a deterrent to quality. Four adverse event measures are used in this study: same day ED visit or hospitalization, 30-day serious gastrointestinal events resulting in ED visit or hospitalization, 30-day other gastrointestinal events resulting in ED visit or hospitalization, and 30-day non-gastrointestinal events resulting in ED visit or hospitalization. Physician ownership status is determined based on a court decision in California in 2007. Data sources include the State Ambulatory Surgery Databases (SASD), State Inpatient Databases (SID), Emergency Department Databases (SEDD), State Utilization Data Files, the Area Resource File (ARF), and HMO/PPO data from Health Leaders. After controlling for confounding factors, the study found that colonoscopy patients treated at a physician-owned ASC had similar odds of experiencing same day ED visit or hospitalization and 30-day non-gastrointestinal events resulting in ED visit or hospitalization as those treated in a hospital-based outpatient facility. But the former had significantly higher odds of experiencing 30-day serious gastrointestinal events and 30-day other gastrointestinal events resulting in ED visit or hospitalization. The results are robust to changes in propensity score adjustment approach and to the inclusion of a lagged quality indicator. They suggest that physician ownership of ASCs was not associated with better quality of care for colonoscopy patients. As more complex procedures are shifted from hospital-based outpatient facilities to ASCs, expanded efforts to monitor and report quality of care will be worthwhile.