A smiling Elizabeth walked unassisted out of King Edward VII Hospital Monday afternoon before saying goodbye to staff. She was then driven away in a motorcade.
"The queen has left the King Edward VII's Hospital having been admitted briefly as part of the assessment of symptoms of gastroenteritis," the palace said in a brief statement issued after her release.
Gastroenteritis is an inflammation of the stomach and/or the intestine, generally caused either by food poisoning or the norovirus, a common winter vomiting bug that affects several hundred thousand Britons every year.
The queen has canceled engagements for this week due to the illness. Norovirus infections are only very rarely dangerous, and doctors who weren't involved with Elizabeth's treatment believe she may have been hospitalized as a precaution.
It was the monarch's first hospitalization in 10 years, but it set off few alarm bells in Britain, where she is seen as being in excellent health. Although not as physically active as she used to be - having reluctantly given up horse riding, one of her favorite hobbies, and cut down on taxing overseas trips - the queen still maintains a demanding schedule.
Unofficial court figures indicate she carried out roughly 400 official engagements in 2012 - ranging from weekly one-on-one meetings with the prime minister to gatherings of 100 or more people.
There were no concerned crowds of wellwishers gathered outside the hospital Monday - only news media keeping an eye out for royal visitors coming to pay their respects.