Cooling is used to bring temperatures back to normal after a brain injury
Brain injured children whose temperature is brought below normal to prevent swelling may be more likely to die or suffer harm, scientists warn.
The study, in the New England Journal of Medicine, says the "hypothermia" treatment may not be worthwhile.
It found more children given the therapy within hours of their accidents died or were left severely disabled.
A UK specialist said cooling was normally used only to bring temperature back to normal levels.
Brain injury following an accident is one of the most common causes of death and disability among children.
The technique works on the principle that cooling the brain can restrict blood flow, both reducing swelling inside the skull, which can cause tissue damage, and the level of oxygen reaching cells around the injury, which has also been linked with the level of damage produced.
The authors of the research said they were "very surprised" by the findings, as early studies in adults had suggested the technique could lower the chance of death or disability.
The children involved in the study were treated in hospitals in the UK, France and Canada.
In all, 102 were given the hypothermia treatment. On average, their body temperature was reduced by four-and-a-half degrees centigrade below the normal level of 37°C/98°F.
Another 103 children just had their temperature reduced to normal.
When the children were followed up six months later, it was found 23 of those given the hypothermia treatment had died, and another nine had a severe disability of were in a permanent vegetative state (PVS).
In the other group, 14 had died and a further nine left with a severe disability or in PVS.
Dr Jamie Hutchison who led the study, said: "The slightly higher mortality represents a worrisome trend, which is why we advise that hypothermia therapy only be used under special circumstances."
Dr Hutchison said their study suggested the treatment might be effective if started earlier, kept going longer, and if patients were re-warmed less swiftly afterwards.
Dr Ian Jenkins, president of the Paediatric Intensive Care Society, said that cooling to the temperatures used in the study was not common practice in the UK.
He said that a recent audit had found that 24% of children were treated using hypothermia, although many would have either been taking part in the study, or being cooled to keep control of a rising temperature following their accident.
"There is far more interest in using cooling after cardiac arrest, in both adults and children, and in neonates starved of oxygen at birth, and there is far more evidence supporting these.
"I don't think anyone in this country would be routinely using it after head injury to the levels in this study."