Professional footballers appear to be at increased risk of a nerve disorder that causes paralysis and death, say researchers.
Trauma may be a factor
A study of 7,000 Italian players showed the condition was five times more common than expected.
It is the same type of motor neurone disease that physicist Steven Hawking has, called ALS.
The research, in New Scientist magazine, has also been published in the journal Brain.
It was prompted by the discovery a few years ago of 33 cases of ALS during an investigation of illicit drug use among 24,000 professional and semi-professional football players in Italy.
With the exception of some rare cases that run in families, the cause of ALS is unknown.
It is the most common form of the motor neurone diseases which collectively affect about 5,000 people in the UK.
The disease is caused by the death of cells - called motor neurones - that control movement in the brain and spinal cord. There is currently no known cure.
Dr Adriano Chiņ and colleagues at the University of Turin looked back at the medical records of footballers who had played in Italy's first or second division between 1970 and 2001.
Based on the normal incidence of ALS, it would be expected that one or fewer of the players had ALS. It was found that five had developed the condition.
Furthermore, the players with ALS had developed it at a much earlier age than is typical for the disorder, at around 40 rather than 60 years.
The researchers suggested that the high risk might be linked to sports injuries, performance-enhancing drugs or exposure to environmental toxins such as fertilizers or herbicides used on football fields, as well as genetic factors.
But equally, it might be that people prone to ALS are drawn to sport, said Dr Ammar Al-Chalabi from London's Institute of Psychiatry.
"There could be some quality in their neuromuscular make-up that not only makes them good at sport, football particularly, but also makes them susceptible to ALS," he said.
Dr Brian Dickie of the Motor Neurone Disease Association, said: "We still don't know what causes this link, or whether it would be reflected in other groups of footballers and sportspeople.
"There is some anecdotal evidence of a link between high levels of physical exercise and an increased risk of developing motor neurone disease.
"However, much more research needs to be carried out before we can draw definite conclusions."