It may be possible to detect autism at a much earlier age than previously thought, according to an international team of researchers.
A study published in Current Biology identified differences in infants' brainwaves from as early as six months.
Behavioural symptoms of autism typically develop between a child's first and second birthdays.
Autism charities said identifying the disorder at an earlier stage could help with treatment.
It is thought that one in every 100 children has an autism spectrum disorder in the UK. It affects more boys than girls. While there is no "cure", education and behavioural programmes can help.
One of the researchers, Prof Mark Johnson from Birkbeck College, University of London, told the BBC: "The prevailing view is that if we are able to intervene before the onset of full symptoms, such as a training programme, at least in some cases we can maybe alleviate full symptoms."
His team looked for the earliest signs of autism in 104 children aged between six and 10 months. Half were known to be at risk of the disorder because they had on older sibling who had been diagnosed with autism. The rest were low risk.
Older children with autism can show a lack of eye contact, so the babies were shown pictures of people's faces that switched between looking at or away from the baby.
Sensors attached to the scalp looked for differences in brain activity.
In low-risk babies, or high-risk babies that did not develop autism, there was a large difference in the brainwaves when looking at each type of image.
There was a much smaller difference in the brainwaves of babies who developed autism.
Prof Johnson said: "It is important to note it is not a 100% predictor. We had babies who flagged up warning signs who did not develop autism."
There were also babies who did develop autism who had low-risk brainwaves. The test would need to be more accurate before it was used routinely.
Prof Tony Charman, Centre for Research in Autism and Education at the Institute of Education, said: "Differences in the use of eye gaze to regulate social interaction are already a well-recognised early feature in many children with autism from the second year of life.
"Future studies will be required to determine whether measurements of brain function such as those used in our study might one day play a role in helping to identify children at an even earlier age."
Christine Swabey from the charity Autistica said: "The hope is that this important research will lead to improved identification and access to services for future generations.
"Ultimately, the earlier we can identify autism and provide early intervention, the better the outcomes will be."
Dr Georgina Gomez-de-la-Cuesta from the National Autistic Society said: "Further research to investigate these differences will eventually lead to earlier recognition of the condition.
"Early intervention is very effective in supporting those with autism, so recognition in infancy can only be beneficial in helping individuals with autism reach their full potential.
"However, this important research is still in its early stages, and larger studies looking at several early markers of autism will be necessary before a robust clinical diagnosis could be possible at such a young age."