Daley had complained several times Friday that he was feeling ill and disoriented during a conference he attended in Arizona as part of his work for the law firm Katten Muchin Rosenman, said his longtime spokeswoman, Jackie Heard. When the conference was over, Daley flew back to Chicago and his family arranged to have an ambulance take him directly to Northwestern Memorial Hospital.
The 71-year-old Daley has maintained an active schedule since leaving office in 2011 and exercises regularly, even while traveling, Heard said.
"Mayor Daley has been a very healthy man and he rarely complains of feeling ill. So for him to complain more than once about not feeling quite right, of course that causes alarm," Heard said.
Daley walked off the plane in Chicago before getting into the ambulance, Heard said.
Daley remained in the hospital's intensive care unit Saturday for "monitoring and evaluation," according to a statement released by his family.
Members of the family were with him and visiting the former mayor between tests, the statement said.
No other information on his condition or treatment was released. A spokeswoman for the hospital would not comment beyond the family's statement.
His hospitalization occurred just hours after Daley's nephew pleaded guilty to involuntary manslaughter in the 2004 death of a man during a fight outside of a Chicago bar.
Heard said she believed Daley already was complaining about feeling ill before he learned of the developments in court Friday and that she does not think the news caused him to feel unwell.
Under the plea deal with a special prosecutor, Richard J. Vanecko was sentenced Friday to 60 days in a county jail followed by 60 days of home confinement.
The investigation by a special prosecutor followed a series of stories in the Chicago Sun-Times that raised questions about whether Chicago police and Cook County prosecutors mishandled the original case. The victim's family contended there was a police cover-up because of Vanecko's relationship to Daley.
Daley has been hospitalized at least two other times since 2000, when he was examined by doctors because of mild chest pains. He was also hospitalized in 2002 along with his wife because they were both suffering from severe stomach flu.
Chicago's longest serving mayor with 22 years in office, Daley is credited with giving the city a face lift with new green spaces, a revived theater district, the transformation of Navy Pier into a colorful playground and the creation of Millennium Park.
He stunned the city when he announced in late 2010 that he would not seek a seventh term. About six months after Daley left office, his wife, Maggie, who had been diagnosed with breast cancer in 2002, died on Thanksgiving Day. Daley was succeeded by Rahm Emanuel, who left his job as White House chief of staff to return to Chicago and successfully run for mayor.
Daley is the son of Chicago's second longest-serving mayor. Richard J. Daley died while in office, at age 74, of a heart attack in 1976.