The researchers said that incidence of brain tumors in rural areas of Sweden was much higher among users of GSM cell phones than among rural residents who were not cell phone users. The rate was also higher than among GSM users in urban areas.

The chance of developing a malignant brain tumor was roughly eight times higher for cell phone users in the Swedish countryside than in urban areas. The risk of developing any brain tumor was four times higher for country dwellers using mobile phones for five years or more, compared with those who did not use the devices.

The study suggests that mobile handsets in rural areas deliver a higher dose of electromagnetic radiation because they have to transmit a stronger signal to distant transmission towers, whereas towers are closer together in urban areas, resulting in phones transmitting a weaker signal.

The study was conducted by Lennart Hardell, a professor of oncology at university hospital in Orebro, Sweden. It was published on today in a British journal, Occupational and Environmental Medicine.

Researchers looked at more than 1,400 adults aged 20 to 80 who had been diagnosed with a malignant or benign brain tumor between 1997 and 2000. The brain cancer patients were compared with a similar number of healthy adults living in the same area. Each group was asked to recall their daily use of mobile and cordless phones.

"Clearly our results support the notion that exposure may differ between geographical areas," Hardell said. "But there is no information on the exact difference between geographical areas."