"I feel great. Take care. Merry Christmas, I'm feeling fine thanks to the hospital. They did a great job," a smiling and waving Giuliani said as he left Barnes-Jewish Hospital en route to New York.
He arrived home in New York late Thursday and spoke to reporters outside his Manhattan apartment building.
"I want to thank my wife, Judith. She's been with me through many different things, and there's one great asset in having a wife that's a nurse: Whenever you have any kind of medical issue, she can calm you, help you," he said, with his wife at his side. "She stayed in touch with everything. I'm doing fine. I'm doing great."
Giuliani, 63, was to spend Friday off the campaign trail. Aides said he would make planned stops in New Hampshire on Saturday and Sunday.
The former New York mayor felt the symptoms while campaigning in Missouri and they soon became worse, campaign spokeswoman Katie Levinson said late Wednesday. She did not describe the symptoms beyond those being commonly associated with the flu.
In a statement Thursday, the campaign said doctors had performed a series of precautionary tests and the results were normal.
Giuliani's wife, Judith, told reporters in New York about the circumstances surrounding his surprise hospital stay.
"A decision was made last night when he had a severe headache and flu-like symptoms on his way home from Missouri to land the plane. EMS then performed a small evaluation and decided that for precautionary measures they would take him to Barnes-Jewish Hospital," she said.
Judith Giuliani said he would have a follow-up visit with his physician.
Giuliani's Thursday schedule was already clear of public appearances before the unexpected stop in St. Louis.
Campaigning Wednesday in Missouri, Giuliani had used a baseball analogy to explain his reasons for targeting the "Show Me" state when other candidates are focused on the early voting states of Iowa and New Hampshire, where Giuliani trails his rivals in polls.
Challenging tradition, Giuliani is devoting more of his attention to the delegate-rich Feb. 5 states some two dozen including New York, California and New Jersey hold primaries and caucuses that day while spending limited time in Iowa and New Hampshire.
The former mayor has been the leader in national polls for much of the year, but recently former Arkansas Gov.