Two stages of tests on 15 patients with an advanced form of the degenerative nerve disease were carried out at a hospital in Créteil, just outside Paris, and the third at Addenbrookes hospital in Cambridge.
French neurosurgeon Stéphane Palfi, who led the French stages said the patients, who were between 48 and 65 years old, developed better coordination and balance, had less muscle twitching and improved speech when they were tested after 12 months.
"In those patients who were first operated upon, the improvement lasted up to four years," he said.
The technique proved safe, the researchers reported.
It aims to reverse the lack of a brain chemical called dopamine, which is essential for motor skills, and entails tucking three genes into a disabled horse virus of the family
The modified virus is then injected directly into a specialised area of the brain, where it infiltrates cells and delivers corrective pieces of DNA, prompting defective brain cells to once again start producing dopamine
Called ProSavin, the British-designed treatment was first tried on lab monkeys.
But the treatment does not address non-motor problems also caused by the disease, according to a commentary by Canadian researcher Jon Stoessl published by The Lancet medical journal, which also published the test results.
Parkinson's disease affects five million people worlwide and 120,000 in France.