Monkey stem cells can repair the brain damage caused by Parkinson's disease, Japanese researchers have shown.
Parkinson's is caused by a chemical imbalance in the brain
The findings, published in the Journal of Clinical Investigation, offer renewed hope of a similar treatment for humans.
Until now, research showing stem cell therapy can work in Parkinson's has mainly been carried out on rodents.
The Kyoto University team said more research was now needed to prove the treatment was safe and effective.
Stem cells are premature cells that are capable of becoming any of a number of mature cells within the body, given the right conditions.
Dr Jun Takahashi and colleagues took some stem cells from monkeys and encouraged them to grow into the brain cells, or neurons, that are damaged in Parkinson's disease.
These are neurons that produce the chemical messenger dopamine.
To encourage their development the researchers exposed the stem cells to a growth factor that is produced exclusively in the area of the brain affected by Parkinson's disease and is thought to have a protective effect on dopamine-producing neurons.
They then transplanted the stem cell-derived dopamine-producing neurons into monkeys with a condition analogous to human Parkinson's disease.
The transplanted cells worked as hoped, and reduced the symptoms of Parkinson's in the monkeys.
The researchers said: "These results suggest that transplantation using embryonic stem cells as a clinical therapy for Parkinson's disease is approaching the point of technical feasibility."
But they said a number of safety and efficacy concerns still needed addressing.
Rodents treated in a similar way went on to develop tumours.
Also, the number of cells produced by the stem cell technique may still be too few to treat humans.
Commenting on the research, Dr J William Langston of the Parkinson's Institute in California, US, said: "While the observations in the current study are encouraging, the number of surviving dopamine-producing neurons was very low.
"It is good news that tumours were not observed, but this could also be related to the small number of surviving cells."