Rising health care costs threaten boomers' retirement security. In 2040, half of adults age 65 and older will spend at least 19 percent of their incomes on health care, up from 10 percent in 2010, if costs grow at the intermediate rate projected by the Medicare trustees. About 7 in 10 older Americans in the bottom two-fifths of the income distribution will spend more than 20 percent of their incomes on health care in 2040. These projections underscore the importance of controlling health care costs and the need for boomers to plan for future health care spending.