Former US President George Bush Snr remains hospitalised near his home in Houston after experiencing shortness of breath this week but is "doing well", his spokesman said.
Bush, 90, was taken by ambulance to Houston Methodist Hospital on Tuesday.
Spokesman Jim McGrath said yesterday that the 41st president's prognosis was positive and that he remained hospitalised as a precaution.
The former Republican president suffers from Parkinson's disease and cannot use his legs, but he celebrated his 90th birthday in June by skydiving with a US Army parachute team.
George Bush Snr is being kept under observation as a "precaution" after struggling to breathe, his spokesman Jim McGrath said.
The 90-year-old was taken by ambulance to Houston Methodist Hospital on Tuesday evening.
It is the same hospital where he spent nearly two months in January 2013 after treatment for a bronchitis-related cough and other health issues.
Bush, who can no longer use his legs, most recently appeared seated in a wheelchair at an event in November at Texas A&M University with his son, former President George W. Bush.
Former US president George Bush Snr has been hospitalised at the age of 90 after experiencing a shortness of breath, a spokesman said.
The former US president George H W Bush has completed a 6,300 ft parachute jump celebrate his 90th birthday.
President Bush's first parachute jump took place when he was shot down over the Pacific island of Chi Chi Jima in 1944.
He says his most recent jump, the eighth he has taken, is to be his last.
Former US First Lady Barbara Bush has been cleared to leave hospital in Houston, Texas after spending nearly a week being treated for a "respiratory-related issue".
A spokesman for Mrs Bush, the wife of former President George H. W. Bush, said she had responded well to medication and was cleared to go home in the afternoon.
Former US President Bill Clinton has tweeted this photo of himself with his predecessor George Bush Snr.
The pair have lunch together every year in the state of Maine, where Mr Bush owns a large estate on the coast.
Mr Clinton drew attention to his host's "western cactus" themed socks, of which he said he was "envious".
The former US president George Bush Sr has celebrated his 89th birthday.
The White House's official Twitter page posted a picture of Mr Bush with Barack Obama.
Mitt Romney, the Republican's presidential candidate for last year's contest tweeted: "Happy birthday to a fellow socks fan!"
Former US President George Bush Sr said: "Barbara and I were deeply saddened to learn of Baroness Thatcher's passing, and extend our heartfelt condolences to her children and loved ones.
"Margaret was, to be sure, one of the 20th Century's fiercest advocates of freedom and free markets - a leader of rare character who carried high the banner of her convictions, and whose principles in the end helped shape a better, freer world.
"The personal grief we Bushes feel is compounded by the knowledge that America has lost one of the staunchest allies we have ever known; and yet we have confidence that her sterling record of accomplishment will inspire future generations.
"May God bless the memory of Margaret Thatcher."