The Orkney Islands have the highest rate of multiple sclerosis in the world, University of Edinburgh scientists said.
Researchers found the rate for probable or definite cases of MS in the Orkneys is now 402 per 100,000 - up from a previous high of 309 per 100,000 in 1974.
Dr Jim Wilson, of the university's Centre for Population Health Sciences, 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."
The study appears in the Journal of Neurology, Neurosurgery & Psychiatry.