OBJECTIVE: To identify the impact of changes surrounding certification as a patient-centered medical home (PCMH) on outcomes for patients with diabetes. STUDY SETTING: Minnesota legislation established mandatory quality reporting for patients with diabetes and statewide standards for certification as a PCMH. Patient-level quality reporting data (2008-2018) were used to study the impact of transition to a PCMH. STUDY DESIGN: Achievement of Minnesota's optimal diabetes care standard-in aggregate and by component-was modeled for adult patients with Type 1 or Type 2 diabetes as a function of time relative to the year the patient's primary care practice achieved PCMH certification. Patients from uncertified practices were used to control for general trend. Practice-level random effects captured time-invariant characteristics of practices and the practices' average patient. DATA COLLECTION: Electronic health record data were submitted by 695 Minnesota practices capturing components of the quality standard: blood sugar control, cholesterol control, blood pressure control, nonsmoking status, and use of aspirin. PRINCIPAL FINDINGS: The first cohort of practices achieving PCMH certification (July 2010-June 2014) showed statistically insignificant changes in optimal care. The next cohort of practices (July 2014-June 2018) achieved larger, clinically meaningful increases in quality of care during the time prior to and following certification. Specifically, this second cohort of practices was estimated to achieve a 12.8 percentage-point improvement (P < .001) in the predicted probability of providing optimal diabetes care over the period spanning 3 years before to 3 years after certification. CONCLUSIONS: Our results suggest that the initial cohort of certified practices was already performing at a high level before certification, perhaps requiring little change in their operations to achieve PCMH certification. The second cohort, on the other hand, made meaningful, quality-improving changes in the years surrounding certification. Differences by cohort may partially explain the inconsistent PCMH impacts found in the literature.