Doctors in Leeds have carried out the NHS's first adult-to-adult live donor liver transplant.
Stephen Lomas and his son David in hospital after the operation
The eight-hour operation - involving two pairs of surgeons working simultaneously in separate theatres - was carried out at St James' Hospital.
The hospital said Stephen Lomas, 51, of Ulverston in Cumbria, was making "excellent progress" after receiving part of his son David's liver.
David, 20, has been discharged since the operation on 21 June.
NHS liver transplants usually rely on organs becoming available after a donor has died.
Consultant hepatologist Dr Charlie Milson said one-in-five liver transplant patients currently died while on the waiting list.
Leeds Teaching Hospitals NHS Trust said the operation was the first of its kind in the NHS and could "hold the key" to saving hundreds of lives a year.
Dr Milson said: "The liver has two important features that make living-related donation possible - it is much larger than we need and it can regenerate within weeks if part of it is removed.
"In this procedure we remove part of the liver of a healthy donor and transplant it into a patient with liver failure.
The live donor liver transplant took eight hours
"The remaining liver in the donor will regrow within weeks to almost its normal size.
"One in five of our liver transplant patients currently die on the waiting list.
"This development means many of these lives will be saved. It really is a massive leap forward."
Live donor liver transplantation has been carried out in other countries for more than 15 years, but it is the first time the treatment has been made available to adult patients in the UK by the NHS.
Son David said: "I'm just relieved that I've been able to help my dad."
And his father Stephen's comment: "He's a top man, a top man."