Medicare's approximately 250 accountable care organizations (ACOs) care for a growing portion of all fee-for-service beneficiaries across the United States. We examined where ACOs have formed and what regional factors are predictive of ACO formation. Understanding these factors could help policy makers foster growth in areas with limited ACO development. We found wide variation in ACO formation, with large areas, such as the Northwest, essentially empty of ACOs, and others, such as the Northeast and Midwest, dense with the organizations. Key regional factors associated with ACO formation include a greater fraction of hospital risk sharing (capitation), larger integrated hospital systems, and primary care physicians practicing in large groups. Area income, Medicare per capita spending, Medicare Advantage enrollment rates, and physician density were not associated with ACO formation. Together, these results imply that underlying provider integration in a region may help drive the formation of ACOs.