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It has become one of the most controversial questions in cancer medicine: Can using a cell phone cause brain tumors? The federal government and the mobile-phone industry have maintained that there is no conclusive data to support a link between cell-phone radiation and cancer, but a growing band of scientists are skeptical, suggesting that the evidence that does exist is enough to raise a warning for consumers — before mass harm is done.

What scientists and regulators need is truly conclusive scientific evidence. Enter Interphone, a $24 million long-term study that matched rates of brain cancer with cell-phone use among more than 12,000 participants (including 7,400 who developed tumors) in 13 countries. The long-awaited report — whose findings were finally released on Monday after years of delay — is by far the most comprehensive look at the issue to date, and was designed to provide the final word on the debate.

Unfortunately, the results turned out to be anything but clear. The study, which will be published this week in the International Journal of Epidemiology, found that, overall, there is no clear connection between cell-phone use and brain cancer. "An increased risk of brain cancer is not established from the data from Interphone," says Dr. Christopher Wild, director of the International Agency for Research on Cancer, which helped coordinate the study.

But upon closer inspection, the results were checkered: the 10% of people who used their phones most often and for the longest period of time — 30 minutes a day or more on average for at least 10 years — had a substantially higher risk of developing some form of brain cancer than those who didn't use a mobile phone at all. Meanwhile, people who used their cell phones infrequently had a lower risk of developing some brain tumors than those who exclusively used corded telephones — as if mobile phones in small doses might offer some protection from brain cancer.

The mobile-phone industry was quick to trumpet Interphone's most basic results. The overall finding "is in accordance with the large body of existing research and many expert reviews that consistently conclude there is no established health risk from radio signals that comply with international safety recommendations," said Jack Rowley, director of research and sustainability for the GSM Association, which represents hundreds of mobile-phone makers and operators, in a statement.

But consumer advocates, who have in the past raised concerns about the safety of mobile phones, counter that the study did find cancer risks for heavy users. More troubling than that, they say, are the apparent methodological problems in the study itself. The fact that all but the heaviest users of cell phones had a smaller risk of developing a brain tumor than those who never used a mobile phone at all almost certainly demonstrates a flaw in the study design. If there had been a true zero risk from mobile-phone use, a well-designed study would have shown no difference in brain-cancer rates between cell-phone users and nonusers. "Bias stands as the most likely explanation of the observed results," wrote epidemiologists Rodolfo Saracci and Jonathan Samet in an editorial accompanying the study.

Further, if cell-phone use posed no increased risk of brain cancer, it doesn't logically follow that heavy users would have a 40% higher incidence of glioma (a certain form of brain tumor) than the control group, as the study found. And it's worth keeping in mind that "heavy" users as defined by Interphone — 30 minutes of cell-phone use a day — are not what we might consider heavy today: average use is now 21 minutes per day. (Interphone's heavy user would require 900 cell-phone minutes a month — the midlevel usage category for AT&T wireless plans.)

Importantly, the study — the results of which are now 5½ years old — does not take into account the impact of mobile-phone use in children, whose skulls are thinner than adults, and who may be more sensitive to cell-phone radiation. Nor does it include any data on U.S. users.

Of course, the study doesn't make a conclusive argument that cell phones cause brain cancer, either. Indeed, there's no scientific agreement on how cell-phone radiation might even biologically cause a tumor. (Mobile-phone radiation is nonionizing, unlike carcinogenic X-rays.) Instead of offering clear answers, Interphone just raises more questions. "Further investigation of mobile-phone use and brain-cancer risk is merited," says Wild. That's one thing the mobile-phone industry and its opponents can agree on.

• Bryan Walsh