Barbara Bush, the wife of former US president George HW Bush, is in "failing health" and has decided to no longer seek medical treatment, the office of the ex-president has said in a statement from Houston.
The former first lady, the statement said, "will instead focus on comfort care". She is 92 years old.
The short statement did not indicate the nature of Mrs Bush's illness but said that she had had a series of recent hospitalisations.
She has been treated for decades for the thyroid condition Graves' disease.
"It will not surprise those who know her that Barbara Bush has been a rock in the face of her failing health, worrying not for herself — thanks to her abiding faith — but for others," the statement said.
"She is surrounded by a family she adores and appreciates the many kind messages and especially the prayers she is receiving."
She is mother of former President George W Bush, 71, the 43rd US president.
Her husband, the 41st US president, is 93 years old.
Mrs Bush's other son is a former governor of Florida who ran unsuccessfully for president in 2015 and early 2016, quitting after a series of lacklustre performances highlighted by tough skirmishes with then-candidate Donald Trump.
With hair that had turned white prematurely, she was known by family members as the "Silver Fox".
Her work as first lady focused on promoting literacy and reading.
At the time, she said she was more interested in running a household than in helping her husband run the country.
Mrs Bush is the only woman in history to see her husband and son sworn in as US president.
Abigail Adams, first lady from 1797 to 1801, was a major influence on husband John Adams, the nation's second president, but died before her son, John Quincy Adams, was elected president in 1824.