While upholding the Affordable Care Act, the Supreme Court ruled that states could opt out of the Medicaid expansion without losing existing federal funding. In this report, we analyze Medicaid enrollment and costs for Washington under a full expansion to 138% FPL, expansion to 100% FPL, and no Medicaid expansion. By 2020, a full expansion would lead to 256,000 more adults and 16,000 more children enrolled than with no expansion. This would lead to $2.1 billion more in total spending on Medicaid. However, the difference in the state share would only be $115 million. Savings in uncompensated care could offset this increase.