Urine could potentially be used for a quick and simple way to test for CJD or "human mad cow disease", say scientists in the journal JAMA Neurology.
The Medical Research Council team say their prototype test still needs honing before it could be used routinely.
Currently there is no easy test available for this rare but fatal brain condition.
Instead, doctors have to take a sample of spinal fluid or brain tissue, or wait for a post-mortem after death.
What they look for is tell-tale deposits of abnormal proteins called prions, which cause the brain damage.