Monkeys given a new hormone-like compound lost a massive seven per cent of their body weight - in just one week.
Could a human version be produced?
Researchers are now hopeful that a drug can eventually be developed to tackle the "obesity epidemic" threatening the West.
The compound, called KB-141, works on the same principle as the hormone produced by the thyroid gland in the neck.
It is well known that giving this to patients can produce weight loss by boosting the metabolic rate - so that the body burns more calories.
However, this comes at a price, as the hormone also causes an potentially-dangerous increase in the heart rate.
Scientists hoped that if they could somehow separate the two effects, then perhaps a drug could be made which would produce the weight loss without the heart problems.
A team of researchers from the Karolinska Institute in Sweden and the University of California, in San Francisco, believe they have found a way to do this.
Thyroid hormone works by stimulating two main receptors on the body's cells.
One of these, called TR-alpha, appears to control heart rate, while the other, TR-beta, appears to regulate the metabolism.
They have developed an experimental chemical called KB-141 which they believe stimulates TR-beta, but not TR-alpha.
Animals given the drug showed decreased cholesterol levels and reduced body weight, but crucially, without a faster heartbeat.
When the compound was given to monkeys, there was a seven per cent drop in body weight over just seven days - with no effect on heart rate.
"Our results in three species show that a separation between potential beneficial versus deleterious effects is possible," say the researchers, writing in the Proceedings of the National Academy of Sciences.
A drug which could control obesity is a "Holy Grail" for researchers, as record levels of the disease are recorded in many western countries.
Drug companies are racing to find ways to stimulate the body to shed weight, with the promise of vast profits for a successful compound.
Other areas of research include "gene therapies" designed to tackle genetic mutations that predispose to obesity, and drugs designed to reduce hunger.
Professor John Lazarus, the president of the British Thyroid Association, said that the finding was "very exciting".
He said that the effects on the heart meant that it was dangerous to give thyroid hormone supplementation to patients unless it was clear their own gland was not making enough.
"This certainly shows promise - the amount of weight loss is impressive, although there are question marks over whether this loss could be sustained in a human patient.
"The research needs to proceed with caution - with longer term studies, probably in monkeys."
He said that it had to be proven that the new compound avoided another damaging side-effect of getting too much thyroid hormone - bone loss.