Great Ormond Street Hospital is officially marking its 100th lung and 200th heart transplant on Tuesday.
Great Ormond Street is a world leader in children's transplants
The famous children's hospital in central London was among the first in the world to carry out the transplants on children in 1988.
It is the biggest centre of its kind in Europe and carries out 30 children's heart and lung transplants a year.
The hospital will use the event to highlight improvements in survival rates and the shortage of donors.
Dr Paul Aurora, a lung specialist at the hospital, told BBC News they wanted to thank the families of organ donors for their "great generosity" at a time of personal crisis.
He added: "When we started in 1988, this was an experimental procedure really, although it had been performed on adults for a few years.
"There were very few centres in the world that were doing heart transplants or lung transplants on children.
"We didn't really know how it was going to work out, but over the last 18 years, as well as having helped all these children, our results have got steadily better."
The hospital's first heart transplant patient in early 1988 was a boy with congenital heart disease. They carried out the operation along with a transplant team from Papworth Hospital, near Cambridge.
The 100th lung and 200th heart transplant patients will join staff to celebrate the milestone on Tuesday.