Former first lady Barbara Bush was "doing great and responding well" on Thursday while undergoing treatment at a Houston hospital for a respiratory-related issue, a family spokesman said.
The wife of former President George H.W. Bush has been hospitalized since Monday at Methodist Hospital in Houston. There was no timetable for her release, but "we're hopeful that will take place sooner rather than later," family spokesman Jim McGrath said.
McGrath said physicians merely may be exercising extra caution by keeping her at the hospital.
"She is 88," he said. "She is a national treasure."
She and her husband, the 41st president, live in Houston. They are the longest-married presidential first couple and will celebrate their 69th wedding anniversary on Jan. 6.
On Thursday, the former president tweeted President Barack Obama and former President Bill Clinton, thanking them on his wife's behalf for their tweets of support the previous day and saying she was "heeding their advice."
"Doesnt happen w every President she knows!" Bush wrote.
The former first lady had a reputation for bluntness when her husband was president. Her son, George W. Bush, was the nation's 43rd president.
The Bush family matriarch had heart surgery in March 2009 for a severe narrowing of the main heart valve. She also was hospitalized in November 2008, when she underwent surgery for a perforated ulcer. In 2010, she was admitted to the hospital after having a mild relapse of Graves disease, a thyroid condition for which she was treated in 1989.
But health concerns had been more common of late with her 89-year-old husband, the nation's oldest living former president. He was released in January 2013 after spending nearly two months at Houston Methodist Hospital, being treated for a bronchitis-related cough and other health issues.
The former first lady and her husband still make public appearances. Last week, they appeared at an event at the former president's office to honor a Houston businessman and philanthropist with a Points of Light Award, a volunteer service award started by George H.W. Bush.