Parkinson's can lead to difficulty in moving arms and legs
There is strong evidence that exposure to pesticides significantly increases the risk of Parkinson's disease, experts believe.
It comes as another study, published in the BMC Neurology journal, has made the link to the neurological disease.
The US researchers found those exposed to pesticides had a 1.6 times higher risk after studying 600 people.
Experts said it was now highly likely pesticides played a key role - albeit in combination with other factors.
The disorder, which normally develops later in life and can affect movement and talking, is also influenced by genetic factors.
Several gene defects have been identified, but these are thought to be rare and only account for a small proportion of the 120,000 people affected by the disease in the UK.
The US team, which involved scientists from Duke University, Miami University and the Udall Parkinson's Disease Research Center of Excellence, quizzed 319 patients about their pesticide use.
The answers were compared to over 200 family members and other controls who did not have the disease.
Related individuals were chosen as they would share many environmental and genetic backgrounds in a bid to isolate the impact of the pesticides.
They found those exposed to pesticides had a 1.6 times greater risk of developing the disease.
Heavy use, classed as over 200 days exposure over a lifetime, carried over double the risk.
And the study also revealed herbicides and insecticides were the pesticides most likely to increase risk.
Lead researcher Dana Hancock said: "I think there is very strong evidence now linking the two. What we need to find out how - the biological process.
"What we noticed in our research was that recreational pesticide use in the home and garden was more of a source of exposure than occupational use."
Kieran Breen, director of research at the Parkinson's Disease Society, said the link had been recognised by earlier studies, but this study "strengthened the fact that pesticides play a key role".
However, he added: "We still don't know exactly what causes Parkinson's. It's most likely to be a combination of genetic susceptibility and environmental factors."
He pointed out a survey of 10,000 patients by the charity had revealed only one in 10 had had long-term exposure to pesticides.