Oestrogen has long been linked to mental illness
A hormone patch may protect women with schizophrenia or other severe mental illnesses from psychotic feelings.
Australian scientists found that women given the sex hormone oestrogen were less likely to report suffering hallucinations or delusions.
Writing in the Archives of General Psychiatry Journal, they said the hormone might be enhancing blood flow to the brain.
A UK mental health charity said funding was now needed for bigger studies.
While men do have some oestrogen in their bodies, it is produced in much greater concentrations in the female body, where, among other functions, it helps regulate the menstrual cycle.
The link between oestrogen and mental illness was first recognised more than a century ago, but only recently has it been considered as a possible treatment.
The researchers from Monash University in Melbourne recruited more than 100 women with diagnosed schizophrenia, half of whom were given a patch containing estradiol - the most common form of oestrogen.
The other half also wore a patch, but with no active drugs, and both groups carried on taking their normal medication.
Over the next month, their symptoms and feelings were recorded on a weekly basis.
The group given estradiol had a greater improvement in psychotic symptoms over that period, and were less likely to report other negative changes in their condition.
The researchers, led by Dr Jayashri Kulkarni, said that the hormone might have a swift effect on blood flow in the brain, and on the way the brain uses sugar as fuel.
However, they said that longer-term effects, which might actually change the way brain cells communicated with each other, were also a possibility.
They said that there might be a role for the hormone in other severe mental illnesses affecting women.
A spokesperson for mental health charity Rethink said: "We welcome any new research that could lead to the development of effective treatments for psychosis - especially if those treatments bring fewer of the horrible side-effects that are associated with some medications for schizophrenia.
"These findings are exciting, but we can't jump the gun - this is one small study and it needs to be followed up with further research before any of us can be really excited about it."
She said that a track record of poor research funding into mental health in the UK could jeopardise any similar research here, even though it could potentially benefit thousands of patients.