Queen Elizabeth II visited her husband, Prince Philip, on Wednesday in the London hospital where he is receiving treatment for a bladder infection.
The Duke of Edinburgh, who will turn 91 on Sunday, was taken to King Edward VII hospital Monday after falling ill during the queen's Diamond Jubilee celebrations over the weekend, including Sunday's rain-swept river pageant.
He missed a pop concert held at Buckingham Palace on Monday and was absent from the queen's side for Tuesday's events, seen as the ceremonial climax of the jubilee festivities.
They included a service of thanksgiving at St. Paul's Cathedral, a carriage procession and a flypast at the palace.
The queen did not speak to the media on her way into the hospital. Prince Andrew later joined his mother inside.
The queen, who spent about 50 minutes at the hospital, had earlier attended a lunch with commonwealth leaders.
A statement on the official British Monarchy website Wednesday said the Duke of Edinburgh's condition had "improved considerably" since he was admitted to the hospital.
"The treatment of his infection continues with antibiotics. He is likely to remain in hospital over the next few days. He is in good spirits," the statement said.
Prince Edward, who visited Prince Philip's hospital room Tuesday, said his father was recovering but needed rest.
He also said the Duke of Edinburgh had watched the day's proceedings on television.
He is expected to remain under observation for several days.
Prince Philip was "disappointed" not to take part in Tuesday's events, the queen's press secretary said in a statement Monday.