US scientists claim a drug can reverse some of the early symptoms of Alzheimer's disease - with the first effects seen within 10 minutes.
An excess of tumour necrosis factor-alpha may cause Alzheimer's
The Journal of Neuroinflammation reports how the memory of an 81-year-old man improved sharply after etanercept was injected into his spine.
His wife described it as her husband being "put back to where he was".
But UK experts warned that a single success did not prove that the drug would work for every dementia patient.
An ageing population means a substantial increase in the numbers of people suffering Alzheimer's disease.
Some studies have suggested that too much of a body chemical called tumour necrosis factor-alpha may be at least partly to blame for the advance of the condition.
Etanercept, which is licensed for use as a rheumatoid arthritis drug, works to block this body chemical.
Scientists from the University of California at Los Angeles, and the University of Southern California, have already published a study which suggested that this could benefit Alzheimer's patients.
In previous studies, they noticed that injecting the drug into the neck spine seemed to deliver almost immediate effects - so set out to test this on just one patient, a former doctor in the early stages of the disease.
Before the drug, they measured his performance on cognitive tests, and he performed poorly, unable to remember the name of the doctor treating him, the date, or the state in which he lived.
He could not perform simple mental arithmetic, or name more than two animals.
Ten minutes after a dose of etanercept, he was noticeably calmer, more attentive, and less frustrated.
He knew he lived in California, and knew the day of the week, and the month.
He could name five animals, and performed better at the arithmetic test.
Interviewed at that point, his wife said that the improvement was "like some kind of science fiction story".
"He's not the same person he was. I see he's clearer, more organised. "There is something has put him back to where he was before. We almost fell off our chairs watching this."
His son said that the change immediately after the drug was administered was the "single most remarkable thing I've seen."
Rebecca Wood, of the Alzheimer's Research Trust, said: "This is promising and innovative research but in the early stages and further work is needed before we can conclude etanercept could work as a treatment for Alzheimer's.
"We need to investigate whether it is safe and works in a larger number of patients as well as monitor the long-term effects.
"Scientists also need to check the benefits weren't just due to the placebo effect and establish whether any benefit is just temporary or whether the disease itself is slowed. "
Neil Hunt, of the Alzheimer's Society, echoed that view: "It is crucial more research is carried out, before any conclusions are drawn on TNF alpha and the development of Alzheimer's disease."