The Queen has been discharged from a London hospital today after being treated for symptoms of gastroenteritis.
Looking relaxed and smiling, she appeared on the steps of King Edward VII's Hospital and walked to her waiting limousine.
The Queen was admitted yesterday for assessment by doctors after being struck down by the stomach bug on Friday.
Her illness forced her to cancel a trip to Rome to meet the country's President Giorgio Napolitano for a private lunch on Thursday.
Being admitted raised concerns about the Queen's health generally as this was the first time the 86-year-old has been treated overnight at hospital in 10 years.
But she looked well and cheerful as she said goodbye to a uniformed member of staff who curtseyed to her.
The Queen, who wore a scarlet coat, was driven away by a chauffeur and is likely to spend time convalescing at her favourite home Windsor Castle.
A Buckingham Palace spokeswoman declined to discuss in detail her condition and just said that she remained in "good spirits".
Gastroenteritis is an infection of the stomach and bowel which leads to vomiting and diarrhoea.
She was struck down by the stomach bug on Friday and admitted to King Edward VII's Hospital in central London yesterday as a "precaution", but is "otherwise in good health", according to a spokesman.
"She travelled by private car. There was never any suggestion at all of an ambulance taking her," a Buckingham Palace spokeswoman said.
The most common causes of gastroenteritis are norovirus - the winter vomiting bug that has swept the country in recent months - and food poisoning.
The infection interferes with one of the intestine's main functions - the absorption of water into the body. This is why the most common symptom of gastroenteritis is watery diarrhoea and why dehydration is a standard complication.
Experts say it is "rare" for people suffering from the illness to be admitted to hospital.
But if someone is admitted with the infection, it is likely that they will stay in hospital between two and five days, said Dr Anton Emmanuel.
"This is a very common problem that most people will have had at some point in their lives, but it very rare to be hospitalised for it," said the consultant gastroenterologist, who works at London's University College Hospital as well as King Edward VII's Hospital where the Queen is being treated.
"Symptoms typically include lots of loose motion and vomiting, abdominal pain and fever. For most, it is self-resolving and not a big deal.
"People only tend to be hospitalised if they are dehydrated or to control symptoms of fever, pain or vomiting.
"In the western world it is very rarely a thing that causes morbidity. Most of the time we only admit people as a precaution."
Professor Christopher Hawkey, at the University of Nottingham's faculty of medicine, said: "The likely cause with the Queen is the norovirus, the winter vomiting virus.
"It's very infectious and strikes in winter because people are indoors and it spreads more easily.
"Because it is infectious, we try to not admit people to hospital as it can start the outbreaks we hear of.
"But not everyone can keep up with oral hydration so it is pretty routine to go to hospital and have a drip and wait for the thing to pass and keep hydrated."
It is important to ensure patients are given combinations of sugar and salt to help fluids be absorbed.
There is an urban myth that drinking a can of flat cola will help as it has the right amount of sugar. But since manufacturers have cut salt out entirely, "it's actually a really bad thing", said the professor.
Most types of gastroenteritis are highly infectious.
Prof Hawkey added that the Queen is likely to be kept in isolation while staying in hospital to prevent the spread of the infection.
Queen Elizabeth's busy shcedule for March
The Queen has a busy month ahead of her, with her annual Commonwealth Day speech set for the start of next week.
Although the monarch, who is being treated in hospital for symptoms of gastroenteritis, has cancelled her engagements this week, including a two-day trip to Rome, Buckingham Palace said it was too early to say whether she would attend forthcoming events.
On Monday, the Queen, who places great importance on her role as head of the Commonwealth, is due to speak in front of 2,000 people at the annual Commonwealth Observance multi-faith service in Westminster Abbey in central London.
She delivers her message each year at the celebration of the Commonwealth and in 2013 is set to speak on the theme of opportunity through enterprise.
After the service, the Queen is due to attend the Commonwealth Reception given by the Commonwealth Secretary-General at Marlborough House.
Since 1977, Commonwealth Day has been celebrated every year on the second Monday in March.
Other engagements coming up include a joint visit to Tech City, east London, on Wednesday March 13 with her son the Duke of York.
On Thursday March 14, the Queen and the Duke of Edinburgh are due at a service at the Guards' Chapel to commemorate the 175th anniversary of the Soldiers and Airmen's Scripture Readers Association.
On Thursday March 28, the Queen is also scheduled to distribute the Royal Maundy at the Maundy Service in Christ Church Cathedral, Oxford.
In April, she turns 87 while Prince Philip's 92nd birthday is in June.
As well as being the birth year of a future King or Queen with the Duke and Duchess of Cambridge's baby due in July, 2013 marks the 60th anniversary of the Queen's coronation, including a service of celebration at Westminster Abbey in June and a coronation festival at the Palace in July. It comes after the hectic diamond jubilee year of 2012.
In November, the Commonwealth Heads of Government Meeting (CHOGM) will take place in Colombo, Sri Lanka.
The Queen has been present at every summit since she first attended in Ottawa, Canada, in 1973.
She missed the first one in 1971 in Singapore when British Prime Minister Edward Heath advised her not to attend because of a row over Britain selling arms to South Africa.