Brain scans can detect autism long before any symptoms start to emerge, say scientists.
The earliest that children tend to be diagnosed at present is at the age of two, although it is often later.
The study, published in the journal Nature, showed the origins of autism are much earlier than that – in the first year of life.
The findings could lead to an early test and even therapies that work while the brain is more malleable.
One in every 100 people has autism, which affects behaviour and particularly social interaction.
The study looked at 148 children including those at high risk of autism because they had older siblings with the disorder.
All had brain scans at six, 12 and 24 months old.