The Osmonds have visited the Children's Hospital for Wales as it became one of the first in the UK to join a charity the singing family helped to launch.
The Children's Miracle Network is said to have raised $4bn for similar hospitals in North America.
Jimmy, Jay and Marie Osmond met the Cardiff hospital's patients and staff and to support its Noah's Ark Appeal.
The Osmonds, who are playing Cardiff International Arena, have found their ancestors came from Merthyr Tydfil.
Seven of the Osmonds - Alan, Wayne, Merrill, Jay, Donny, Marie and Jimmy - are touring to celebrate 50 years performing together as a family.
The fact that we're Welsh - and the fact that we're starting here - is a real honour for our family
The fame of the Utah-based family peaked in the 1970s, when Donny, Marie and Jimmy also all had individual success.
They co-founded the Children's Miracle Network 25 years ago. It now has 175 partner hospitals in the US and Canada, and last year helped 17m children needing treatment.
The network is backed by giant corporations including Microsoft and Walmart, and the Children's Hospital for Wales is confident it will boost the Noah's Ark Appeal.
Marie, who made her first visit to Merthyr on Tuesday, said she was glad to be supporting a hospital in Wales.
"The fact that we're Welsh - and the fact that we're starting here - is a real honour for our family."
Appeal chairman Lyn Jones said it was an exciting day.
The Osmond family at their peak in unmistakeable 1970s style
"They've got all these contacts. They'll pass them on to us and we'll follow them through and I'm sure we'll get a fantastic hospital because of their commitment and the commitment of the people of Wales."
Each year, the hospital will nominate a champion child to go to the Children's Miracle Network's festival at Disneyland in Florida.
Champion children and their families are also flown to Washington to meet the US president.
While the young patients who met the Osmonds in Cardiff may not have been too familiar with their music, their parents and many of the hospital staff certainly were.
"I remember their Saturday programmes when they were really big stars, and it's just nice that they're supporting the children," said play specialist Jayne Williams.
Angela Mills, whose 11-year-old foster son Raymond has had major spinal surgery, said: "I don't think they've changed much in the years.
"They're still lovely, and Marie doesn't look a day older either."
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