Patients with multiple sclerosis (MS) are more than twice as likely to have abnormal blood flow from the brain as healthy people, according to initial results from a trial.
Scientists said they were "cautiously optimistic" about the research, which suggests a link between narrowing of veins in the head and neck and the neurological condition.
More than half (56.4%) of MS patients who took part in the research were found to have narrowing of the extracranial veins, which restricts normal blood flow, compared with 22.4% of healthy controls.
The results emerged from preliminary data of a randomised clinical study at the University of Buffalo in the US.
They are based on the first 500 participants in the Combined Transcranial and Extracranial Venous Doppler Evaluation (CTEVD) study, which began last April.
Complete data will be presented at the American Academy of Neurology meeting in April.
Robert Zivadinov, associate professor of neurology and principal investigator on the study, said he was "cautiously optimistic and excited" about the results.
He said: "The data encourage us to continue on the same course.
"They show that narrowing of the extracranial veins, at the very least, is an important association in multiple sclerosis.
"We will know more when the MRI and other data collected in the CTEVD study are available."
The researchers want to find out if chronic cerebrospinal venous insufficiency (CCSVI) is a major risk factor for MS.
The theory is that the complex vascular condition restricts the normal flow of blood from the brain, altering the blood flow patterns in the brain and damaging brain tissue and neurons.
The first 500 participants, which included 280 MS patients, underwent ultrasound (Doppler) scans of the head and neck to view the direction on blood flow.
Those with MS also had MRI scans of the brain to measure iron deposits.
Around 100,000 people in the UK suffer from MS, an auto-immune disease which destroys the fatty insulating sheath of myelin that coats nerve fibres in the brain and spinal cord.
Dr Doug Brown, biomedical research manager at the MS Society, said: "These results are intriguing but it is important to remember that although people with MS may show evidence of CCSVI in screening studies, there's no proof as yet that this phenomenon is a cause of MS, nor that treating it would have an effect on MS.
"The next step is to determine what this actually means for MS, and an investigation into whether there's any potential therapeutic benefit from treatment will be pivotal for this novel theory."