Background: Physicians naturally form networks. Networks could

form a rational basis for Accountable Care Organizations (ACOs) for defined populations of Medicare beneficiaries. Objectives: To use methods from network science to identify naturally occurring networks of physicians that might be best suited to becoming ACOs. Research Design, Subjects, and Measures: Using nationally representative claims data from the Medicare program for CY 2006 on 51 hospital referral regions (HRRs), we used a network science– based community-detection algorithm to identify groups of physicians likely to have preestablished relationships. After assigning patients to networks based upon visits with a primary care physician, we examined the proportion of care delivered within communities and compared our results with potential ACOs organized around single hospitals. Results: We studied 4,586,044 Medicare beneficiaries from 5