The Queen Mother was taken to King Edward VII hospital in central London Wednesday. A day earlier, she had withdrawn from a public engagement because doctors said she had a mild case of heat exhaustion. She has since returned to her residence Clarence House and exited the hospital aided only be a cane.
"The treatment has been completed satisfactorily," her office said.
The spokeswoman declined to give further details but British newspapers said the transfusion had taken up to eight hours to complete.
Prime Minister Tony Blair, in Mexico on an official visit, welcomed the good news.
"The Queen Mother is a hugely popular figure, and not just in Britain, but around the world. I am sure we all want her to make a speedy recover and enjoy her 101st birthday," he said during a news conference with Mexican President Vicente Fox.
The Queen Mother was reportedly eager to be at home in time for her birthday celebrations. Traditionally, she goes to the gates of Clarence House to greet well-wishers who gather outside.
She then joins the rest of the royal family for a celebration lunch while military guns boom out salutes across London.
The normally sprightly Queen Mother, mother to ruling Queen Elizabeth and a symbol of wartime resistance against Nazi Germany, has kept up a busy round of engagements despite her age from visits to flower shows to horse racing events.
A newspaper quoted unnamed royal sources last month as saying the Queen Mother had told house guests that she wanted to become Britain's oldest woman.
Although doctors had diagnosed her with anemia, aides were quick to downplay concern over the centenarian's health.
The Queen Mother's private secretary Sir Alistair Aird said, "There is no cause for alarm."
Anemia is a deficiency of hemoglobin, the oxygen-carrying pigment of red blood cells. Sufferers generally feel weary and look pale.
The latest complication compounds health problems faced by the Queen Mother in recent years, including hip surgery that has made walking difficult.
In 70 years of service, the "Queen Mum" has remained a popular figure while other royals fell in and out of favor.
After her self-effacing husband unexpectedly took the throne in 1936, replacing his brother who had abdicated, she worked tirelessly to uphold the royal family's reputation and popularity as the old order and class barriers crumbled.
She became a key focus for morale during World War Two, defying German bombers by staying in London through the Blitz. Adolf Hitler described her as the most dangerous woman in Europe for her efect on public spirits.
©MMI, CBS Worldwide Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press and Reuters Limited contributed to this report