Mark Ronson made the dreams of nine seriously ill children come true as they joined him in the studio to record a special version of his hit Uptown Funk.
The music producer admitted his nerves as he led the group of 6 to 17-year-olds, who were all having their musical wish granted by Rays of Sunshine Children's Charity, and later sat down for one-on-one sessions with them.
He said: "I've never had any experience like that of leading a choir, I was like, 'Oh, what if I don't know how to talk to kids', but everyone was so good, their enthusiasm led the whole thing anyway.
"We really only did one or two run-throughs at the piano and then all the kids got in front of the mic and killed it, it was so great.
The studio session at Metropolis Studios in Chiswick, West London was made all the more poignant as Mark said the last time he had been there was to record the strings for Amy Winehouse's hit Valerie.
The 40-year-old praised the talent of Amy fan Alfie Sharp, 13, from Nottingham, who was diagnosed last year with Acute Myeloid Leukemia, and who sat down at the piano to play him a cover of Me And Mr Jones.
Alfie and aspiring rapper Dylan Berry Billbrough, 17, from Teesside, stole the show with their respective soulful solo and self-penned rap as the group recorded their version of the Bruno Mars track.
Fifteen-year-old Lexie Edwards, from Sutton, who lost her eye to a rare childhood cancer, admitted she had only begun practising the day before as she played chords on her electric guitar, which Ronson later signed for her.
She said: "I sat down him and I played the guitar while he played the drums and it was really cool. He said, 'Wow, you've really got the rhythm for this song'.
"It would be amazing to be a musician, really hard, but that would be the dream."
Ronnie Deville, 10, who had only received the call to come that morning, and Murad Shaikh, 17, laid the beat on the drums.
Murad, who was born with renal failure, said: "It was a great experience. I told my friends who promote concerts and they were so jealous."
The enthusiastic choir was completed by Harvey Spanner, 10, from Fareham, Eden Taverner-Wright, six, from Welwyn Garden City, Tia-Mae Nicholson, nine, from Strood, and Ella Chadwick, eight, from Lancashire.
Mark was put in his place when he accidentally started singing with the children during the chorus and apologised.
"You just try your best," quipped Ella, who was celebrating the two year anniversary of her second kidney transplant, after her first one, when she was three years old, failed.
Her mum Karen said: "As soon as she came in she just started singing her heart out. It's great to come to London and it not to involve a hospital."
Uptown Funk went to number one in the UK for seven weeks and Mark joked the kids knew all the words because it was "so overplayed".
The day was organised by Rays of Sunshine, which grants wishes for seriously and terminally ill children to distract from their medical conditions.