Many thousands of adults in the UK may have autism without knowing it, research suggests.
A survey conducted in England found that around 1% of adults met the diagnostic criteria for an autistic spectrum disorder.
But not a single one of those identified realised they were affected by the condition.
Autism covers a range of developmental handicaps, including Asperger's syndrome, which impair a person's ability to communicate with and relate to others.
Individuals with autism can be affected in a wide range of different ways. While some need full time support, others manage to live almost undisturbed by the condition.
The research found autism was more common in men, people without higher educational qualifications, and those living in social housing.
Scientists carrying out the survey, the first study of autism prevalence in a general adult population, conducted 7,461 screening interviews in 2007.
The new findings, published today in the journal Archives of General Psychiatry, confirmed previously reported data showing that 9.8 per 1,000 adults in England had autism.
They revealed no evidence of an "autism epidemic" or increasing rates of the condition.
But the study did show that none of the identified cases of autism had previously been diagnosed.
Lead researcher Professor Traolach Brugha, from the University of Leicester, said: "It is very concerning that none of the cases we confirmed using rigorous diagnostic assessment methods in the community knew that they had the condition or had an official diagnosis.
"As in all community surveys it is of course likely that most of the cases we found were relatively mild and few were severe. We know that severe autism particularly when accompanied by learning disability is much more likely to be recognised. We are beginning to provide training to psychiatrists in the diagnosis of autism spectrum conditions in adulthood through the Royal College of Psychiatrists Education and Training Centre, London."
Applied to the UK as a whole, a 1% prevalence rate would mean around 600,000 adults have diagnosable symptoms of autism.
The research suggests very many of them will not know they suffer from the condition.
Initial findings from the survey were first reported in 2009 and attracted considerable media attention.