Some cases of obsessive compulsive disorder in children may be a result of an immune reaction following an infection, scientists believe.
Many people with OCD wash their hands compulsively
Researchers found children with OCD were more likely to have antibodies associated with streptococcal infection than those without the disorder.
But the joint Institute of Psychiatry and Institute of Neurology team said more research was needed.
The findings were reported in the British Journal of Psychiatry.
About 3% of the population suffers from the anxiety disorder, which is generally treated by drugs or cognitive behaviour therapy counselling.
Researchers tested the blood of 50 children with OCD for the presence of anti-basal ganglia antibodies, which are produced when antibodies raised in response to a streptococcus infection react with part of the brain.
Such an immune response is closely linked to movement disorders, such as Sydenham's chorea, which themselves are linked to OCD.
The team found that 42% of the OCD children had the antibodies, compared to just 5% of the 190-strong control group.
Writing in the British Journal of Psychiatry, the authors said the findings were significant and suggested that "autoimmunity may have a role in the genesis and/or maintenance of some cases of OCD".
And they added: "Further examination of this autoimmune subgroup may provide insight into the neurobiology of OCD, and explain whether the antibodies concerned are causing the disease."
But they admitted further research was needed to confirm their findings as evidence suggested genetics also played a role in the development of the disorder.
Ashley Fulwood, chief executive of support group OCD UK said: "The truth is that we just do not know what causes it.
"Some people say it is hereditary, others that environmental factors play a role.
"According to people I talk to, the infection theory does not carry much weight, but in the end what we need is more research."