Spending 50 minutes with a mobile phone plastered to your ear is enough to change brain cell activity in the part of the brain closest to the antenna.
But whether that causes any harm is not clear, say scientists at the US National Institutes of Health (NIH), adding the study isn't likely to settle recurring concerns of a link between mobile phones and brain cancer.
"What we showed is glucose metabolism (a sign of brain activity) increases in the brain in people who were exposed to a mobile phone in the area closet to the antenna," says Dr Nora Volkow of the NIH, whose study was published in the Journal of the American Medical Association (JAMA).
The study was meant to examine how the brain reacts to electromagnetic fields caused by wireless phone signals.
Volkow says she was surprised that the weak electromagnetic radiation from mobile phones could affect brain activity. But she say the findings do not shed any light on whether the phones cause cancer.
"This study does not in any way indicate that. What the study does is to show the human brain is sensitive to electromagnetic radiation from [mobile] phone exposures."
Use of the devices has increased dramatically since they were introduced in the 1980s, with about 5 billion mobile phones now in use worldwide.
Some studies have linked mobile phone exposure to an increased risk of brain cancers, but a large study by the World Health Organization was inconclusive.
Volkow's team studied 47 people who had brain scans while a mobile phone was turned on for 50 minutes and another while the phone was turned off.
While there was no overall change in brain metabolism, they found a 7 per cent increase in brain metabolism in the region closest to the phone antenna when the phone was on.
Interpret with caution
Experts says the results are intriguing, but urges they be interpreted with caution.
"Although the biological significance, if any, of increased glucose metabolism from acute cell phone exposure is unknown, the results warrant further investigation," write Henry Lai of the University of Washington, Seattle, and Dr Lennart Hardell of University Hospital in Orebro, Sweden, in a commentary in JAMA.
"Much has to be done to further investigate and understand these effects," they write.
Professor Patrick Haggard of University College London says the results are interesting since the study suggests a direct effect of mobile phone signals on brain function.
But he says much larger fluctuations in brain metabolic rate can occur naturally, such as when a person is thinking.
"If further studies confirm that mobile phone signals do have direct effects on brain metabolism, then it will be important to investigate whether such effects have implications for health," he says.
Volkow says the findings suggest the need for more study to see if cell phones have a negative effect on brain cells.
Meanwhile, Volkow isn't taking any chances. She now uses an ear phone instead of placing a mobile phone next to her ear.
"I don't say there is any risk, but in case there is, why not?"