Under the Patient Protection and Affordable Care Act (PPACA), Medicaid enrollment is expected to grow by 16 million people by 2019, an increase of more than 25 percent. Given the unwillingness of many primary care physicians (PCPs) to treat new Medicaid patients, policy makers and others are concerned about adequate primary care capacity to meet the increased demand. States with the smallest number of PCPs per capita overall--generally in the South and Mountain West--potentially will see the largest percentage increases in Medicaid enrollment, according to a new national study by the Center for Studying Health System Change (HSC). In contrast, states with the largest number of PCPs per capita--primarily in the Northeast--will see more modest increases in Medicaid enrollment. Moreover, geographic differences in PCP acceptance of new Medicaid patients reflect differences in overall PCP supply, not geographic differences in PCPs' willingness to treat Medicaid patients. The law also increases Medicaid reimbursement rates for certain services provided by primary care physicians to 100 percent of Medicare rates in 2013 and 2014. However, the reimbursement increases are likely to have the greatest impact in states that already have a large number of PCPs accepting Medicaid patients. In fact, the percent increase of PCPs accepting Medicaid patients in these states is likely to exceed the percent increase of new Medicaid enrollees. The reimbursement increases will have much less impact in states with a relatively small number of PCPs accepting Medicaid patients now because many of these states already reimburse primary care at rates close to or exceeding 100 percent of Medicare. As a result, growth in Medicaid enrollment in these states will greatly outpace growth in the number of primary care physicians willing to treat new Medicaid patients.