George HW Bush has been admitted to an intensive care unit to address a respiratory problem from pneumonia, according to a spokesman for the former US president.
Jim McGrath said Mr Bush's wife, Barbara Bush, has also been admitted to hospital as a precaution after experiencing fatigue and coughing.
Mr McGrath said the 92-year-old former president was placed under sedation for a procedure to protect and clear his airway.
He is stable and resting comfortably. He has been at Houston's Methodist Hospital since Saturday.
Barbara Bush is 91. The couple marked their 72nd wedding anniversary on January 6.
Mr Bush was taken to the hospital over the weekend after suffering from shortness of breath. Mr McGrath had earlier said on Wednesday that he was responding well to treatments.
"Doctors and everyone are very pleased, and we hope to have him out soon," Mr McGrath said.
Mr Bush's chief of staff, Jean Becker, told the Houston Chronicle that the 41st US president was expected to go home in a couple of days.
Mr Bush, who served as US president from 1989 to 1993, has a form of Parkinson's disease and uses a motorised scooter or a wheelchair for mobility.
He was admitted to hospital in Maine in 2015 after falling at his summer home and breaking a bone in his neck. He was taken into hospital in Houston the previous December for around a week because of shortness of breath. Mr Bush spent Christmas 2012 in intensive care for a bronchitis-related cough and other issues.
Despite the loss of mobility, Mr Bush celebrated his 90th birthday by taking part in a tandem parachute jump in Kennebunkport, Maine.
Last summer, Mr Bush led a group of 40 injured veterans on a fishing trip at the helm of his speedboat, three days after his 92nd birthday celebration.
Mr Bush's office announced earlier this month that he and his wife of more than 70 years would not attend President-elect Donald Trump's inauguration this week due to the former president's age and health.
George Herbert Walker Bush, born on June 12 1924, in Milton, Massachusetts, also served as a congressman, CIA director and Ronald Reagan's vice president.
His son, George W Bush, was elected president in 2000 and served two terms. Another son, Jeb - a former Florida governor - made an unsuccessful bid for the Republican nomination in 2016.
Only one other US president, John Adams, had a son who also became president.