It's real, doctors say. At last they have proof. The good news is that sufferers have a specific chemical signature in their blood, similar to changes seen in hibernating animals.
Symptoms include extreme mental and physical tiredness and painful limbs but in extreme cases, some have had to quit their job and retire to their bed or end up in a wheelchair.
Today, the cause is still unknown and patients are often dismissed as malingerers. And that's why this new discovery is so important.
Researchers at the University of California, San Diego, analysed blood samples taken from 45 people with CFS and 39 healthy individuals of the same age. This revealed a distinct set of chemical changes that were only present in those with CFS.
Lead researcher Robert Naviaux said something similar happens when animals hibernate and in CFS, the body may get stuck in this state which leads to chronic fatigue. He said the blood test is more than 90 per cent accurate in diagnosing this condition.
Symptoms vary and were similar to other diseases, so the blood test will speed up the correct diagnosis.
But Andrew McIntosh, an Edinburgh University psychiatrist described the new discovery as 'intriguing' and cautioned that much more work needed to be done before the test is suitable for widespread use.
"Nevertheless, it's a promising start" he said.