Psychotic illnesses such as schizophrenia may be linked to immune system disorders, research has found - opening up new avenues for treatment.
Scientists at the University of Oxford found that up to one in 11 patients may have developed psychosis as a result of a condition where antibodies attack the brain.
The nationwide study analysed blood samples from 228 people, with nine per cent testing positive for the relevant antibodies.
Lead researcher Prof Belinda Lennox, a clinical psychiatrist at the university, said the study could have major implications for treatments.
However, others treated the results with caution - voicing doubt that there could be thousands of people in institutions who have been misdiagnosed.
Josep Dalmau, from the University of Barcelona, was among those to express scepticism.
"The large majority of patients develop other symptoms, for example seizures or abnormal movements," he said.
"These usually bring the diagnosis to the doctors, so I think (considering) the number of patients you may have in a psychiatric institution with well-established psychotic disorder for several months or years, the likelihood they have these antibodies is extremely low."