During the last year, about 300 sufferers from Parkinson's disease have received transplants of cells into their brains. The grafts come either from their own adrenal glands, or from aborted foetuses.
Some surgeons have claimed dramatic improvements in their patients. Others say they have seen no change.
Georgina Ferry examines the scientific evidence, and considers the ethical questions surrounding the introduction of new treatments.
Producer DEBORAH COHEN