Some young children accurately diagnosed as autistic lose their symptoms and their diagnosis as they get older, say US researchers.
The findings of the National Institutes of Health study of 112 children appears to challenge the widely held belief that autism is a lifelong condition.
While not conclusive, the study, in the Journal of Child Psychology and Psychiatry, suggests some children might possibly outgrow autism.
But experts urge caution.
Much more work is needed to find out what might explain the findings.
Dr Deborah Fein and her team at the University of Connecticut studied 34 children who had been diagnosed with autism in early childhood but went on to function as well as 34 other children in their classes at school.
On tests - cognitive and observational, as well as reports from the children's parents and school - they were indistinguishable from their classroom peers. They now showed no sign of problems with language, face recognition, communication or social interaction.
For comparison, the researchers also studied another 44 children of the same age, sex and non-verbal IQ level who had had a diagnosis of "high-functioning" autism - meaning they were deemed to be less severely affected by their condition.
It became clear that the children in the optimal outcome group - the ones who no longer had recognisable signs of autism - had had milder social deficits than the high-functioning autism group in early childhood, although they did have other autism symptoms, like repetitive behaviours and communication problems, that were as severe.
The researchers went back and checked the accuracy of the children's original diagnosis, but found no reason to suspect that they had been inaccurate.
Label for life?
The researchers say there are a number of possible explanations for their findings.
It might be that some children genuinely outgrow their condition. Or perhaps some can compensate for autism-related difficulties.
Dr Thomas Insel, director of the National Institute of Mental Health, said: "Although the diagnosis of autism is not usually lost over time, the findings suggest that there is a very wide range of possible outcomes.
"Subsequent reports from this study should tell us more about the nature of autism and the role of therapy and other factors in the long term outcome for these children."
It could be that autism cannot always be accurately defined or diagnosed, particularly since the condition affects people in different ways.
Indeed, experts have disagreed about what autism is.
The American Psychiatric Association is currently revising its diagnostic manual - the "bible" for doctors that lists every psychiatric disorder and their symptoms.
Its new version proposes changes he UK's National Autistic Society says could affect the way diagnoses will be given to people on the autism spectrum.
Instead of using the current terms of autistic disorder, Asperger's disorder, childhood disintegrative disorder and PDD-NOS (pervasive developmental disorder not otherwise specified), people will be given an umbrella diagnosis of "autism spectrum disorder".
And their impairments will be reduced to two main areas - social communication/interaction and restricted, repetitive patterns of behaviour, interests, or activities.
Most diagnoses in the UK are based on the International Classification of Diseases (ICD), published by the World Health Organization, which is up for revision in 2015.
According to the National Autistic Society, more than one in every 100 people, more than 500,000 people in all, in the UK have autism.
About a fifth, an estimated 106,000, are school-aged children.
Dr Judith Gould, director of the National Autistic Society's Lorna Wing Centre for Autism, said: "Autism is a lifelong disability affecting the way that people communicate and interact with others.
"This study is looking at a small sample of high functioning people with autism and we would urge people not to jump to conclusions about the nature and complexity of autism, as well its longevity.
"With intensive therapy and support, it's possible for a small sub-group of high functioning individuals with autism to learn coping behaviours and strategies which would 'mask' their underlying condition and change their scoring in the diagnostic tests used to determine their condition in this research.
"This research acknowledges that a diagnosis of autism is not usually lost over time and it is important to recognise the support that people with autism need in order to live the lives of their choosing."
She said getting a diagnosis could be a critical milestone for children with autism and their families, often helping parents to understand their children better and helping them to support their children in reaching their full potential.