King Charles has been admitted to hospital for treatment for an enlarged prostate, says Buckingham Palace.
The procedure is being carried out at the London Clinic private hospital, where Catherine, the Princess of Wales had surgery last week.
It's understood the King visited his daughter-in-law in the hospital on Friday morning, ahead of his own scheduled treatment.
The King is expected to spend at least one night at the hospital.
Queen Camilla was seen with the monarch when he arrived at the hospital in Marylebone in central London.
King Charles had made his health problem public as a way of sending a message to other men to get their prostates checked.
Following the disclosure, the NHS website saw a surge in searches about enlarged prostates, an interest welcomed by doctors and charities.
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.
Around 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 won't 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.

When she leaves the hospital it is expected that she will spend several months recuperating at home in Windsor. It means the princess will not be taking part in any official duties until after Easter.
There was a third royal health story this week, when it was revealed that Sarah Ferguson, the Duchess of York, had been diagnosed with malignant melanoma, a form of skin cancer.
It had been discovered during treatment for her breast cancer, which had been diagnosed last summer.
"Naturally another cancer diagnosis has been a shock, but I'm in good spirits and grateful for the many messages of love and support," said the duchess, who was formerly married to Prince Andrew.
A spokesman for the duchess said she will have further investigations to ensure the melanoma has been caught in the early stages.
Latest Stories
-
Atiwa East DCE urges teamwork to drive development and attract investment
28 seconds -
Ɔdadeɛ Y2K group marks 25th anniversary with sustainable kitchen project at PRESEC
19 minutes -
New ECG board can ensure improvement – IPP Boss
29 minutes -
Our DCE is not involved in galamsey – Asaman Tamfoe Chief and residents defend Ernest Owusu Ntim
32 minutes -
Spare parts dealers instructed to slash prices to reflect cedi gains
1 hour -
Ɔdadeɛ Y2K group highlights key projects to mark 25th anniversary
1 hour -
Cedi Appreciation: “Gold for oil” not the same as “Gold for reserves”
1 hour -
Reform Ghana school feeding programme – Eduwatch tells Gender Minister
1 hour -
Omane Boamah leads Ghana to join Burkina Faso in honoring Sankara and Rawlings at historic memorial ceremony
4 hours -
I was 20 when I played for Ghana’s U-17 team – Charles Taylor
6 hours -
JoySports Editor Fentuo Tahiru launches Tumu Community Cup set for May 24
6 hours -
Joseph Taylor breaks Ghana’s 29-Year 400m national record with sub-45s run
7 hours -
Greater Accra public lands: Government to set up Commission of Inquiry
7 hours -
Cleanliness to be used for measuring performance of MMDCEs – Mahama
7 hours -
You are now a beacon of hope – Nungua Mantse to President Mahama
7 hours