Former US President George H.W. Bush is in intensive care after suffering from pneumonia, a family spokesman has said.
His wife, Barbara, the former First Lady, has also been hospitalised as a precaution after experiencing fatigue and coughing.
Bush, 92, has been in hospital since Saturday after experiencing shortness of breath. The oldest-living ex-president was sedated during a procedure to "protect and clear his airway" following an "acute respiratory problem stemming from pneumonia", his office said.
It added that he is "resting comfortably" at Houston Methodist Hospital and will remain there for observation.
Mrs Bush was admitted to the same hospital on Wednesday morning.
Mr Bush is the father of former President George W. Bush and former Florida Governor Jeb Bush, who sought the 2016 Republican presidential nomination.
The elder Bush, a Republican like his sons, was vice president during Ronald Reagan's two White House terms before being elected president in 1988. He lost his re-election bid to Bill Clinton, then the Democratic governor of Arkansas, in 1992.
A naval pilot during World War Two, Bush became an occasional skydiver after leaving the presidency and marked his 90th birthday with a jump out of a helicopter.
He has used a wheelchair in his later years and was hospitalised twice in 2014, once for seven weeks with pneumonia and again for breathing difficulties.
In July 2015, he broke a bone in his neck in a fall at the family home in Maine.
Bush was already planning to miss President-elect Donald Trump's inauguration on Friday due to health reasons.
Outgoing president Barack Obama opened his final news conference at the White House this afternoon by sending his prayers to the couple, who he said had been a "constant source of friendship and support."
"They have not only dedicated their lives to this country, they have been a constant source of friendship and support and good council for Michelle and me over the years," President Obama said.
"They are as fine a couple as we know and so we want to send our prayers and our love to them."