The 88-year-old former first lady was admitted to a Houston hospital on Monday. A spokesman for her husband's office says her condition has not changed, with Mrs. Bush in "great spirits" and getting visits from the former president and her family.
Bush had heart surgery in 2009 for narrowing of the main heart valve. She also spent time in a hospital in 2010 after having a mile relapse of Graves disease, a thyroid condition.
President Barack Obama is among those saying he hopes Bush gets well soon.
Bush and her husband, the 41st president, live in Houston and still make public appearances. Last week, they honored a Houston businessman and philanthropist with a Points of Light Award, a volunteer service award started by the former president.
December 31st at 9:00 p.m.: Statement by the Office of George H. W. Bush:
"Former First Lady Barbara Bush was admitted to Methodist Hospital in Houston's Texas Medical Center yesterday for treatment on a respiratory related issue. She is in great spirits, has already received visits from her husband and family, and is receiving fantastic care. Updates will be issued when warranted."