TOKYO (AFP) - Part of a Japanese woman's lung was transplanted to her three-year-old son on Monday in what was described as the world's first successful graft of a middle lobe from a living donor, a hospital said.

Lung transplants from living donors usually involve transferring the inferior lobe which has greater breathing capacity.

But a middle lobe was transplanted in this case as it is smaller than an inferior lobe and is of the right size for the boy, the Okayama University Hospital said.

The boy and his mother were not identified.

The transplant of a middle lobe is seen as difficult, said the university in western Japan.

"A pump-oxygenator was detached from the recipient and he started breathing with the transplanted part of the lung," the hospital said in a statement.

"We deem that the operation has been successful," it said.

The hospital said earlier that a successful operation would be the first of its kind in the world.

The surgery started at 1.35pm and the boy started breathing with the transplanted lung about five hours later, the statement said.

The boy is the youngest lung recipient in Japan, the hospital said.

He underwent a bone-marrow transplant for leukaemia about two years ago but later developed graft-versus-host disease, a complication in which the newly transplanted material attacks the recipient's body, Kyodo News said.

Dr Takahiro Oto, associate professor of respiratory surgery at the state-run university hospital, said the transplant of a middle lobe would pave the way for saving the lives of babies who have not been able to undergo other types of lung transplant, Kyodo reported.