Under the Affordable Care Act (ACA), states can expand Medicaid eligibility for nonelderly adults up to 138 percent of the federal poverty level (FPL). As of May 2016, 19 states had not expanded Medicaid. We estimate that another 4.8 million uninsured people would gain Medicaid eligibility if the remaining states were to expand Medicaid. If all of the 19 remaining states were to expand Medicaid in 2017, we project that the number of uninsured would fall by an additional 4.1 to 5.0 million, depending on enrollment success.