The Affordable Care Act of 2010 will expand Medicaid to millions of Americans by 2014. How many enroll will greatly affect health care access, demand for clinicians, and the federal budget, yet the precision and validity of enrollment estimates made to date is unknown. We created a simulation model using two nationally representative data sets to determine the range of reasonable projections, estimating eligibility, participation, and population growth using prior research and our data. Our model predicted that the number of additional people enrolling in Medicaid under health reform may vary by more than 10 million, with a base-case estimate of 13.4 million and a possible range of 8.5 million to 22.4 million. Estimated federal spending for new Medicaid enrollees ranged from $34 billion to $98 billion annually, and we projected that 4,500-12,100 new physicians will be needed to care for new enrollees. In the end, Medicaid enrollment will be determined largely by the extent to which federal and state efforts encourage or discourage eligible people from enrolling. Yet our results indicate that policy makers should prepare to handle a broad range of contingencies and uncertainty in Medicaid expansion under health reform.