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BACKGROUND The United States has 2 types of degree programs that educate physicians: allopathic and osteopathic medical schools. OBJECTIVE To determine whether quality and costs of care differ between hospitalized Medicare patients treated by allopathic or osteopathic physicians. DESIGN Retrospective observational study. SETTING Medicare claims data. PATIENTS 20% random sample of Medicare fee-for-service beneficiaries hospitalized with a medical condition during 2016 to 2019 and treated by hospitalists. MEASUREMENTS The primary outcome was 30-day patient mortality. The secondary outcomes were 30-day readmission, length of stay (LOS), and health care spending (Part B spending). Multivariable regression models adjusted for patient and physician characteristics and their hospital-level averages (to effectively estimate differences within hospitals) were estimated. RESULTS Of 329 510 Medicare admissions, 253 670 (77.0%) and 75 840 (23.0%) received care from allopathic and osteopathic physicians, respectively. The results can rule out important differences in quality and costs of care between allopathic versus osteopathic physicians for patient mortality (adjusted mortality, 9.4% for allopathic physicians vs. 9.5% [reference] for osteopathic hospitalists; average marginal effect [AME], -0.1 percentage point [95% CI, -0.4 to 0.1 percentage point]; P = 0.36), readmission (15.7% vs. 15.6%; AME, 0.1 percentage point [CI, -0.4 to 0.3 percentage point; P = 0.72), LOS (4.5 vs. 4.5 days; adjusted difference, -0.001 day [CI, -0.04 to 0.04 day]; P = 0.96), and health care spending ($1004 vs. $1003; adjusted difference, $1 [CI, -$8 to $10]; P = 0.85). LIMITATION Data were limited to elderly Medicare patients hospitalized with medical conditions. CONCLUSION The quality and costs of care were similar between allopathic and osteopathic hospitalists when they cared for elderly patients and worked as the principal physician in a team of health care professionals that often included other allopathic and osteopathic physicians. PRIMARY FUNDING SOURCE National Institutes of Health/National Institute on Aging.