The Orkney Islands have, per capita, more people with the neurological condition than any other place, according to a study looking at the prevalence of the disease across the world.
The finding lends weight to the theory that the absence of strong sunlight may be a factor in being diagnosed with MS.
The research also discovered that the number of people with MS in Orkney has increased, and that one in 170 women has the condition.
Researchers said the study, carried out with the universities of Edinburgh and Aberdeen, is the first of its kind in nearly 40 years.
Now they are trying to work out why the rate in Orkney is so high. Previous studies also show high rates of MS in Orkney, along with other parts of Scotland, Canada, and Scandinavia, but the new research finds that the rate for probable or definite cases in Orkney is now 402 per 100,000.
That figure is up on the previous high, recorded in 1974, of 309 per 100,000 and compares with 295 per 100,000 in Shetland and 229 per 100,000 in Aberdeen.
Dr Jim Wilson, of the Centre for Population Health Sciences at the University of Edinburgh, said: “Our study shows that Orkney has the highest prevalence rate of MS recorded worldwide.
“These findings may reflect improved diagnostic methods, improved survival or rising incidence. We are trying to work out why it is so high, but it is at least partly to do with genes.”
Many experts believe the high prevalence of MS is linked to an absence of strong sunlight, which is needed to make vitamin D in the body. Some scientists and campaigners have lobbied for mass dosing of vitamin D in Scotland.
MS causes myelin, a layer of material that insulates nerve cells in the brain, to break down, which weakens the messages sent through nerves cells from the brain to other parts of the body.
This can cause numbness, visual loss, fatigue, dizziness, and muscle weakness, which can cause disability.
The study appears in the Journal of Neurology, Neurosurgery & Psychiatry.