The director of the University of Pittsburgh Cancer Institute, Dr. Ronald B. Herberman, issued the unprecedented warning to his faculty and staff on July 23.
The warning is contrary to numerous studies that don't find a link between cancer and cell phone use, and a public lack of worry by the U.S. Food and Drug Administration.
Herberman is basing his alarm on early-unpublished data.
He says it takes too long to get answers from science and he believes people should take action now - especially when it comes to children.
"Really at the heart of my concern is that we shouldn't wait for a definitive study to come out, but err on the side of being safe rather than sorry later," the China Daily quoted Herberman, as saying.
In the memo he sent to about 3,000 faculty and staff, Herberman said that children should use cell phones only for emergencies because their brains are still developing.
Adults should keep the phone away from the head and use the speakerphone or a wireless headset, he said.
He even warned against using cell phones in public places like a bus because it exposes others to the phone's electromagnetic fields.
A driving force behind the memo was Devra Lee Davis, the director of the university's center for environmental oncology.
"The question is do you want to play Russian roulette with your brain," she said in an interview from her cell phone while using the hands-free speakerphone as recommended.
"I don't know that cell phones are dangerous. But I don't know that they are safe," she added.