Monkeys with Parkinson's have shown significant improvements after being injected with human stem cells, according to a research team from Yale, Harvard and the University of Colorado.
Stem cells were injected into five of eight monkeys with advanced Parkinson's. Those who received the stem cells improved progressively through the entire post-treatment period and were said to be significantly different from those who were not given the treatment.
Eugene Redmond, lead author of the study, said: "Not only are stem cells a potential source of replacement cells, they also seem to have a whole variety of effects that normalise other abnormalities.
"The human neural stem cells implanted into the primates survived, migrated and had a functional impact."
Mr Redmond went on to explain that, as a treatment, stem cell therapy for humans may be years away.
"It's an important step, but there are a number of studies that need to be done before determining if this would be of any value in clinical settings," he said.
The stem cells were from fetal brain rather than embryos.