ObjectiveTo examine the association between 10-year temporal changes in the primary care workforce and Medicare beneficiaries' outcomes. Data Sources2001 and 2011 American Medical Association Masterfiles and fee-for-service Medicare claims. Study Design/MethodsWe calculated two primary care workforce measures within Primary Care Service Areas: the number of primary care physicians per 10,000 population (per capita) and the number of Medicare primary care full-time equivalents (FTEs) per 10,000 Medicare beneficiaries. The three outcomes were mortality, ambulatory care-sensitive condition (ACSC) hospitalizations, and emergency department (ED) visits. We measured the marginal association between changes in primary care workforce and patient outcomes using Poisson regression models. Principal FindingsAn increase of one primary care physician per 10,000 population was associated with 15.1 fewer deaths per 100,000 and 39.7 fewer ACSC hospitalizations per 100,000 (both p<.05). An increase of one Medicare primary care FTE per 10,000 beneficiaries was associated with 82.8 fewer deaths per 100,000, 160.8 fewer ACSC hospitalizations per 100,000, and 712.3 fewer ED visits per 100,000 (all p<.05). ConclusionsMedicare beneficiaries' outcomes improved as the number of primary care physicians and their clinical effort increased.