Affordable Care Act reforms could reduce the number of underinsured US adults by 70 percent.

To provide a baseline and assess the potential of changes brought about under the Affordable Care Act, this study estimates the number of US adults who were underinsured or uninsured in 2010. Using indicators of medical cost exposure relative to income, we find that 44 percent (81 million) of adults ages 19-64 were either uninsured or underinsured in 2010-up from 75 million in 2007 and 61 million in 2003. Adults with incomes below 250 percent of the federal poverty level account for sizable majorities of those at risk of becoming uninsured or underinsured. If reforms succeed in increasing the affordability of care for people in this income range, we could expect a 70 percent drop in the number of underinsured people and a steep drop in the number of uninsured people.