The first evidence that drug therapy can slow the underlying cause of dementia has been revealed.
The study of an antibody called solanezumab shows that it reduces the decline in brain function of patients in the early stages of Alzheimer's disease by around 30%.
Patients were given an infusion of the antibody once a month for one-and-a-half years.
By then there was a marked difference in the progression of the disease compared to those who did not take the drug.
Positive results would be a pivotal moment in the fight against the disease, as existing drugs only tackle the symptoms, making daily living tasks a little easier.
Dr Clare Walton, research manager for the UK's Alzheimer's Society, said: "After a decade of no new treatments and several failed trials, today's news is really positive."
The manufacturer of the drug, Eli Lilly, had abandoned a trial in 2012 after disappointing results.
However, when it re-analysed the data to look at patients at the earliest stages of Alzheimer's, there were encouraging signs that patients benefited.
Dr Eric Siemers, medical director of Eli Lilly, said: "The point of this is to understand that it does make a difference if you start treatment early not later.
"Every day with these neurodegenerative diseases, there's increasing loss of brain cells.
"This tells us we don't want to wait to start those treatments to slow down the progression of the disease."
Solanezumab targets deformed proteins called amyloid, that accumulate in the brains of people with Alzheimer's disease.
The abnormal proteins form sticky clumps between nerve cells, damaging and eventually killing the brain cells.
The drug is not a cure and patients do not appear to improve on treatment.
However, delaying the onset of dementia by five years would reduce the number of cases by a third, and because it largely affects elderly people, they are likely to die of another cause before the disease becomes apparent.
The data is due to be released at the Alzheimer's Association International conference in Washington.
