The former President underwent an emergency procedure to clear his airways after being rushed to hospital with pneumonia, his office said on Wednesday.
His wife, former first lady Barbara Bush, also was admitted to the same hospital on Wednesday as a precaution after experiencing fatigue and coughing.
George Bush senior will remain in hospital for the inaugeration
And in a shocking, but short and frank, letter to the President-elect, Mr Bush announced he would be unable to join Donald Trump on his big day.
Instead he would be staying home on doctor’s orders as he and his wife recover.
Mr Bush experienced an "acute respiratory problem stemming from pneumonia"
Mr Bush wrote a short and frank letter to the President-elect
He said: "My doctor says if I sit outside in January, it likely will put me six feet under. Same for [wife] Barbara.
"So I guess we're stuck in Texas."
He went on: "But we will be with you and the country in spirit. I want you to know that I wish you the very best as you begin this incredible journey of leading our great country.
“If I can ever be of help, please let me know."
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My doctor says if I sit outside in January, it likely will put me six feet under. Same for [wife] Barbara.
George Bush senior's letter to Donald Trump
Trump had been critical of George W and Jeb during the 2016 presidential campaign and their father did not endorse Trump in his race against Democrat Hillary Clinton.
But in his letter, Bush said he and his wife would be with Trump in spirit at the inauguration and wished him "the very best".
Bush, who at 92 is the nation's oldest living ex-president, has been at Houston Methodist Hospital since Saturday after experiencing shortness of breath, according to family spokesman Jim McGrath.
Mr Bush Senior failed to endorse Mr Trump in the run up to the election
Since then, Bush experienced an "acute respiratory problem stemming from pneumonia" and was sedated for the unspecified procedure.
He will remain in the hospital's intensive-care unit for observation, his office said.
The 91-year-old former first lady, Barbara Bush, has also been admitted to hospital as a precaution after suffering from a coughing attack.
The couple marked their 72nd anniversary on January 6.