Fort Lauderdale, Fla. ? People who have used cell phones for at least 10 years may have an increased risk of developing a rare brain tumor, according to a study published Wednesday in the international journal Epidemiology.

A team of researchers at Institute of Environmental Medicine at the Karolinska Institute in Stockholm, Sweden, found almost a four-fold increase of the tumors — known as acoustic neuromas — on the side of the head where the phone was most often held.

The work was done as part of the World Health Organization’s cell phone research agenda, and experts in the field said it must be taken seriously and was likely to rekindle consumer worries about the risks of using the phones.

“The Karolinska researchers are respected around the world, and this study will force health agencies to take a fresh look at mobile phone risks,” said Louis Slesin, publisher of Microwave News, who has been covering the industry since its early days. “This study should put an end to the industry’s call to stop mobile phone health research.”

At least one past study conducted for the cell phone industry also had suggested a link between the phones and this type of tumor. But cell phone industry officials on Wednesday said the Swedish research was just one study and that no conclusions could be drawn from it.

The study, involving 150 acoustic neuroma patients and 600 healthy people, is one of at least six studies that have investigated possible links between cell phone use and acoustic neuromas. Most of those studies had fewer long-term users than the Karolinska study.

Acoustic neuromas are slow-growing, benign tumors that develop on a nerve linking the brain and the inner ear. The most common first symptom is hearing loss, but as the tumor grows, it can push against brain tissue. If not treated, it can be life-threatening. Such tumors are very rare, occurring in about one person per 100,000 in the general population.

“It’s a natural place to look (for a problem) because this is the area of the head that is exposed,” said Anders Ahlbom, director of the Institute of Environmental Medicine at the Karolinska Institute in Stockholm. When a cell phone is in use, it emits radio-frequency radiation, some of which is absorbed in areas of the head closest to the handset.

To conduct the three-year study, the Karolinska researchers interviewed people who had developed the tumors — asking about their cell phone use, how many different phones they had used, the make and model, duration of calls, whether they used a hands-free set and on which side of the head they held the phone.

Researchers said they found no association between the tumors and the amount of use measured in hours or cumulative number of calls, but rather on the length of time those in the study had been regular users of cell phones. Regular use was defined as an average of at least once a week during six months or more.

Ahlbom said in a phone interview that the data were strong and statistically significant, but the findings must be confirmed by follow-up studies. He said the mechanism by which cell-phone radiation might cause tumors remained unknown.

Dr. David Savitz, chairman of the department of epidemiology at the University of North Carolina School of Public Health, Chapel Hill, said the new findings “suggest something a little bit troublesome.”

“It is significant in the sense that it is the first well-designed study to show this,” Savitz said. “There was an earlier study that came out but it didn’t have as many people with long-term use.”

Dr. Henry Lai, research professor of bioengineering at the University of Washington in Seattle, also said the Karolinska study was not the first to show a link between cell phones and acoustic neuromas.

“Another Swedish researcher, Dr. (Lennart) Hardell found similar results in 2002,” Lai said, “so this is, in effect, a replication. I think the data are quite solid and are cause for concern on long-term cell phone use.”

At least three federal agencies, the Food and Drug Administration, the Federal Communications Commission, and the Environmental Protection Agency each has some role in regulating radio frequency radiation, but only recently has the federal government committed funds to studying the cell phone issue, and those studies are not expected to be completed for five to seven years.

Dr. George Carlo, an epidemiologist then working at George Washington University School of Medicine, coordinated the industry-supported project that began in the mid-1990s. When the money ran out in 2000, Carlo said more research was needed because one study showed the risk of acoustic neuroma was 50 percent higher in people who used cell phones for six years or more, and that there appeared to be a correlation between brain tumors on the right side of the head and the use of the phones on that side.