The results of a major study into mobile-phone use and cancer were released this week, but media interpretation of the findings has varied wildly.

One British newspaper, theDaily Telegraph, stated that the study had "found people who speak on their handset for more than half an hour a day over 10 years are at greater risk of brain cancer". Reporting on the same work, the French news wire AFP said that the study showed "no clear link to brain cancer".

Nature looks at the results of a study that has led to such contradictory reports.

What is this research? Haven't I heard it all before?

The study at the centre of the media storm is an international collaboration called INTERPHONE, run by the International Agency for Research on Cancer in Lyon, France, part of World Health Organization. It is a case-control study, in which people with certain types of cancer were interviewed about their mobile-phone usage. This information was correlated with data from interviews of similar people without these cancers.

Responding to concerns about the health effects of using mobile phones, which are known to emit low-level radiofrequency electromagnetic fields, the study hoped to establish whether usage of mobile phones increased the risk of the two main types of brain cancer -- glioma and meningioma. It involved 2,708 people with glioma, 2,409 with meningioma and 7,658 matched controls.

Some results from the 13 individual countries collaborating in INTERPHONE have previously been released, but the latest paper in the International Journal of Epidemiology is the first to combine results from all 13 nations involved. The study received some funding from mobile-phone companies, but this was governed by guarantees of INTERPHONE's complete scientific independence.

What did the study find?

Unfortunately, the results from this study are not entirely straightforward. "Overall, no increase in risk of glioma or meningioma was observed with use of mobile phones," the study says.

Regular phone use seemed to actually decrease the risk of the cancers when the numbers were crunched. But the 10% of participants who reported the greatest amount of time spent on their mobile phones seemed to have a 40% increased risk of glioma. However, the study points out that there are reasons to doubt these apparent associations.

So using my phone protects me against cancer? Or gives me cancer?

On completing their analysis, the researchers found that being a regular user of a mobile phone seemed to reduce the risk of glioma or meningioma by around 20%. But Anthony Swerdlow, an epidemiologist at the Institute of Cancer Research in London who was involved in the UK arm of the study, says that this result is highly likely to be down to problems that were inherent in the study design.

"We have evidence that the people who refused to be controls are people who didn't use phones," says Swerdlow. This meant that the control group, consisting of people without cancer, was rather skewed, appearing to have more mobile-phone use than would be found in a representative sample from the general population. "The controls were over-represented with phone users," he adds.

Equally, some of those individuals in the top 10% of reported phone usage gave what Swerdlow calls "incredibly implausible values", such as an average of 12 hours of mobile use per day, every day.

Studies to validate the data-collection methods used in INTERPHONE found that asking participants about the number of calls they had made provided more accurate information than asking about how much time participants spent on the phone. When researchers analysed the number of calls made, the top 10% of participants showed no increased risk of cancer.

What about previous research?

Some animal and cell studies have apparently shown an increase in cancer from mobile-phone-type radiation, but these findings have proved difficult to replicate.

"Generally, the biological and cellular testing is inconclusive. There's really no consistent evidence or explicable mechanism by which effects can be seen," says Patricia McKinney, an epidemiologist at the University of Leeds, who also worked on the UK arm of the INTERPHONE study.

Studies in humans have generally not managed to establish any link between mobile-phone use and cancer. Swerdlow notes that one group in Sweden has found an increased risk, but he says that this is an "outlier in the literature".

In addition, despite the ubiquity of mobile phones, there seems to have been no increase in the total number of brain tumours reported in statistics since the advent of the mobile phone.

Do questions remain to be answered?

Undoubtedly. Even the researchers involved in the trial do not all agree on the meaning of their work, with some apparently urging a cautious approach and suggesting that people should limit their phone use.

McKinney says, "There hasn't been complete agreement. We consider we're among the majority that is suggesting no risk."

Furthermore, INTERPHONE studied the effects of mobile-phone usage only in adults. It provides no information about the risk of children using mobile phones, which is one of the more controversial areas.

Equally, because of the time period of the study, no information is available on cancer risk for mobile-phone exposures longer than 15 years. The study notes that "the possible effects of long-term heavy use of mobile phones require further investigation".

So what's the bottom line about a causal link between mobile-phone use and cancer?

No link has been established.

"There are standard criteria for assessing whether data from epidemiological studies show causality or not," says Swerdlow. "The results for this study don't get close to passing the standard tests for whether the results show causation."