IMPORTANCE: Starting in 2006, the Centers for Medicare & Medicaid Services (CMS) has restricted coverage of bariatric surgery to hospitals designated as centers of excellence (COE) by 2 major professional organizations. OBJECTIVE: To evaluate whether the implementation of the COE component of the national coverage decision was associated with improved bariatric surgery outcomes in Medicare patients. DESIGN, SETTING, AND PATIENTS: Retrospective, longitudinal study using 2004-2009 hospital discharge data from 12 states (n = 321,464 patients) of changes in outcomes in Medicare patients undergoing bariatric surgery (n = 6723 before and n = 15,854 after implementation of the policy). A difference-in-differences analytic approach was used to evaluate whether the national coverage decision was associated with improved outcomes in Medicare patients above and beyond existing time trends in non-Medicare patients (n = 95,558 before and n = 155,117 after implementation of the policy). MAIN OUTCOME MEASURES: Risk-adjusted rates of any complication, serious complications, and reoperation. RESULTS: Bariatric surgery outcomes improved during the study period in both Medicare and non-Medicare patients; however, this change was already under way prior to the CMS coverage decision. After accounting for patient factors, changes in procedure type, and preexisting time trends toward improved outcomes, there were no statistically significant improvements in outcomes after (vs before) implementation of the CMS national coverage decision for any complication (8.0% after vs 7.0% before; relative risk [RR], 1.14 [95% CI, 0.95-1.33]), serious complications (3.3% vs 3.6%, respectively; RR, 0.92 [95% CI, 0.62-1.22]), and reoperation (1.0% vs 1.1%; RR, 0.90 [95% CI, 0.64-1.17]). In a direct assessment comparing outcomes at hospitals designated as COEs (n = 179) vs hospitals without the COE designation (n = 519), no significant differences were found for any complication (5.5% vs 6.0%, respectively; RR, 0.98 [95% CI, 0.90-1.06]), serious complications (2.2% vs 2.5%; RR, 0.92 [95% CI, 0.84-1.00]), and reoperation (0.83% vs 0.96%; RR, 1.00 [95% CI, 0.86-1.17]). CONCLUSIONS AND RELEVANCE: Among Medicare patients undergoing bariatric surgery, there was no significant difference in the rates of complications and reoperation before vs after the CMS policy of restricting coverage to COEs. Combined with prior studies showing no association of COE designation and outcomes, these results suggest that Medicare should reconsider this policy.