Enactment of the Patient Protection and Affordable Care Act (ACA) has triggered an important debate over the likely impact on the U.S. health care system of covering upwards of 33 million uninsured people.  A critical missing piece in this debate is a detailed analysis of local-area estimates of the coverage expansion populations and their proximity to primary care providers. We utilize a novel small-area estimation method that incorporates data on insurance coverage, income and geographic location from the Survey of Income and Program Participation (SIPP), the American Community Survey (ACS) and the Current Population Survey (CPS) to produce county estimates of the ACA’s Medicaid expansion population. We utilize this information to identify the size of the potentially eligible population with and without access to coverage (based on state decisions on Medicaid expansion).  We then link our estimates with ZIP-level state registry data on the active primary care workforce – including physicians, nurse practitioners, and physician assistants – and identify “hot spots” where the size of the coverage expansion population, relative to the local primary care workforce, indicate the potential for access to care problems. In particular, we utilize our methodology to relate local measures of the Medicaid expansion eligibles to the active primary care workforce, federally qualified health center capacity, and key county health characteristics.