Her latest stay in the hospital came just two days after being released following surgery to replace a metal rod in her leg that strengthens bone weakened by cancer.
Nothing from her doctors indicated Mrs. Daley's current condition is serious.
Maggie Daley checked herself in to the hospital complaining of flu-like symptoms. She was feeling nauseous and dehydrated.
Mrs. Daley was previously released from the hospital on Tuesday after a 10-day stay during which doctors replaced a titanium rod in her right leg. Some bones in her leg and other parts of other body have been weakened during her battle with breast cancer, which was diagnosed in June of 2002. She has undergone numerous surgeries and treatments since that time.
Mayor Richard M. Daley on Thursday had a busy morning with numerous events, including one welcoming a national summit on city design.
The mayor's office issued a statement which included comments from her doctor, Steven Rosen, who said her condition has remained stable and nothing changed relative to her cancer. The statement said doctors did a battery of tests and decided to keep her for observation.