With health insurance moving toward greater patient cost sharing, this study finds a sharp increase in the number of underinsured people. Based on indicators of cost exposure relative to income, as of 2007 an estimated twenty-five million insured people ages 19-64 were underinsured-a 60 percent increase since 2003. The rate of increase was steepest among those with incomes above 200 percent of poverty, where underinsurance rates nearly tripled. In total, 42 percent of U.S. adults were underinsured or uninsured. The underinsured report high levels of access problems and financial stress. The findings underscore the need for policy attention to benefit design, to assure care and affordability.