People with ME - or chronic fatigue syndrome - have major differences in their blood compared with healthy individuals, experts are suggesting.
University of Edinburgh researchers said their findings could pave the way for a "game-changer" diagnostic test.
Prof Chris Ponting said: "For so long people with ME/CFS have been told it's all in their head. It's not. We see it in their blood."
However some critics of the study said researchers had "overstated" the significance of their work.
The study used data from the UK Biobank – a health database of over half a million people – to compare 1,455 ME/CFS patients with 131,000 healthy individuals.
