Scientists say they have identified the earliest signs of Parkinson's disease in the brain, 15 to 20 years before symptoms appear.
Scans of a small number of high-risk patients found malfunctions in the brain's serotonin system, which controls mood, sleep and movement.
The King's College London researchers say the discovery could lead to new screening tools and treatments.
Experts said larger studies and more affordable scans were needed first.
Parkinson's is a progressive neurological condition affecting about 145,000 people in the UK.
The main symptoms are shaking, tremors and stiffness but depression, memory and sleep problems are also common.
Traditionally, the disease is thought to be linked to a chemical called dopamine, which is lacking in the brains of people with the condition.
Although there is no cure, treatments do exist to control symptoms - and they focus on restoring dopamine levels.
But the KCL research team, writing in Lancet Neurology, suggest that changes in the brain's serotonin levels come first - and could act as an early warning sign.
The researchers looked at the brains of 14 people from remote villages in southern Greece and Italy who all have rare mutations in the SNCA gene, making them almost certain to develop the disease.
Half of this group had already been diagnosed with Parkinson's and half had not yet shown any symptoms, making them ideal for studying how the disease develops.
By comparing their brains with another 65 patients with Parkinson's and 25 healthy volunteers, the researchers were able to pinpoint early brain changes in patients in their 20s and 30s.
These were found in the serotonin system, a chemical which has many functions in the brain, including mood, appetite, cognition, wellbeing and movement.