King Charles has had his treatment for an enlarged prostate, after being admitted to hospital on Friday morning.
Queen Camilla said the King is “doing well”. He is expected to spend at least one night at the hospital.
The procedure was carried out at the London Clinic private hospital, where Catherine, the Princess of Wales had surgery last week.
The King visited his daughter-in-law in the hospital on Friday morning, ahead of his own treatment.
Queen Camilla left the hospital in Marylebone in central London on Friday evening, having arrived with the King this morning.
Before leaving the hospital, she said of her husband: “He’s doing well, thank you.”
King Charles had made his health problem public as a way of sending a message to other men to get their prostates checked.
A statement from Buckingham Palace said the King was “delighted to learn that his diagnosis is having a positive impact on public health awareness”.
He also thanked those who had “sent their good wishes”.
The plan for King Charles, aged 75, to have “corrective procedure” for his enlarged prostate was revealed by the palace last week. He travelled on Thursday from Sandringham to London to have the operation.
A benign prostate problem, which is non-cancerous, is common in older men, according to the NHS.
About one in three men over the age of 50 will have some symptoms of an enlarged prostate, which is a gland that sits just below the bladder.
The medical treatment is not significant enough to require any constitutional changes for the King’s role as head of state. There will not be a need for “counsellors of state” who can stand in when a monarch is seriously ill.
A spokeswoman for Prime Minister Rishi Sunak said he had sent best wishes to the King for a “speedy recovery”.
The King is being treated in the same hospital where his daughter-in-law Catherine had “abdominal surgery” last week.
