Approximately 1 in 5 adults in the U.S.—43.8 million, or 18.5%—experiences mental illness in a given year. Approximately 1 in 25 adults in the U.S.—9.8 million, or 4.0%—experiences a serious mental illness in a given year that substantially interferes with or limits one or more major life activities. GET THE HELP YOU NEED.