A Swedish study suggests that people who use a cell phone (search) for at least 10 years might increase their risk of developing a rare benign tumor along a nerve on the side of the head where they hold the phone.

In an interview Thursday with The Associated Press, one of the researchers behind the preliminary study, Anders Ahlbom (search), said the results were surprising and more research is needed.

Several previous studies have investigated whether the use of cell phones is linked to an increased risk of brain tumors (search). Although experiments haveby Ahlbom and Maria Feychting, professors at the Karolinska Institutet in Stockholm, focused on 750 Swedes who had used cell phones for at least 10 years. It was published in the International Journal of Epidemiology.

In the study, researchers questioned 150 patients already diagnosed with acoustic neuroma, a benign tumor on the auditory nerve that takes several years to grow before being diagnosed, and 600 who did not have it, about their cell phone use.

All 750 subjects had been using cell phones for at least 10 years, nearly all early analog models that emit more electromagnetic radiation than the digital models now on the market. Digital phones emit radiation in pulses; the older analog varieties emit continuous waves. Since cell phones exploded in popularity in the late 1990s, most of those sold used digital technology.

"At the time the study was conducted, only analog mobile phones had been in use for more than 10 years and therefore we cannot determine if the results are confined to use of analog phones or if the results would be similar after long-term use of digital phones," the report said.

The risk of developing a tumor was almost double for those who started to use phones before their diagnosis. In addition, the tumor risk was almost four times higher on the side of the head where the phone was held, Ahlbom and Feychting said.

Retrospective questionnaires are not considered the most accurate method of determining a link between behavior and disease. Many links that emerge from such studies turn out not to be true under more rigorous study.

Acoustic neuroma tumors, which can affect hearing, occur in less than one adult per 100,000 people annually. The tumor pushes on the surface of the brain, but doesn't grow into the brain itself, according to the Atlanta-based Acoustic Neuroma Association.

The study was funded by the European Union and is part of the wider Interphone study coordinated by the International Agency for Research on Cancer.

Previous studies, including one by Finnish scientists in 2002, found that electromagnetic radiation emitted by phones can affect brain tissue, but others have said that's not the case.

The wireless industry has always maintained there is no link between mobile phones and cancer.

The Wireless Association in Washington, D.C., a trade group representing American cell phone manufacturers, urged more research. "The wireless industry agrees that more research is needed in this area to provide definitive answers to any questions that might still exist," it said in a statement Wednesday.

Ahlbom conceded more research was needed, adding the study was not an excuse to avoid using cell phones. "You could say also, of course, if that someone is concerned about these results the easy way to avoid any risk is to use a handsfree set," he said.