Brain scans may allow detection of dyslexia in pre-school children even before they start to read, say researchers.
A US team found tell-tale signs on scans that have already been seen in adults with the condition.
And these brain differences could be a cause rather than a consequence of dyslexia - something unknown until now - the Journal of Neuroscience reports.
Scans could allow early diagnosis and intervention, experts hope.
The part of the brain affected is called the arcuate fasciculus.
Among the 40 school-entry children they studied they found some had shrinkage of this brain region, which processes word sounds and language.
They asked the same children to do several different types of pre-reading tests, such as trying out different sounds in words.
Those children with a smaller arcuate fasciculus had lower scores.
It is too early to say if the structural brain differences found in the study are a marker of dyslexia. The researchers plan to follow up groups of children as they progress through school to determine this.
Lead researcher Prof John Gabrieli said: "We don't know yet how it plays out over time, and that's the big question.
"We do not know how many of these children will go on to develop problems. But anyway, we want to intervene before that, and the younger you do that the better. We already know that reading programmes and interventions can really help."
In the study, the volume of the left arcuate had a particularly strong link with poorer pre-reading test results.
The left arcuate fasciculus connects an area of the brain involved in speech production with another used to understand written and spoken language.
A larger and more organised arcuate fasciculus could aid in communication between those two regions, the researchers say.
Prof Gabrieli said: "This brain area fits with a lot of what we already know. So it's a good candidate."
A few years ago, US doctors described the case of a child who developed dyslexia after radiation treatment for a brain tumour. The same brain region - the arcuate fasciculus - was involved.
A spokeswoman for the British Dyslexia Association said brain imaging was providing "increasing evidence" of notable differences between the brains of people with and without dyslexia.
"It is particularly exciting to envisage a future where this technology could be part of a cluster of indicators that would identify a risk of dyslexic difficulties," she said.
But she said there needed to be far more research to determine if in the future it might be possible to diagnose dyslexia with a brain scan