Smoking could substantially increase the risk of developing multiple sclerosis, say scientists.
Smoking can cause other serious illnesses
A study of 22,000 Norwegians found smokers were 1.8 times more likely to develop MS than non-smokers.
The increase in risk was greater in men than in women, said researchers from the University of Bergen, writing in the journal Neurology.
Experts believe a combination of factors, genetic and environmental, may be to blame for the damaging illness.
MS happens when the immune system launches an attack the protective sheath around nerve cells, causing periods of fatigue and weakness as this interferes with the passage of nerve signals.
These attacks can leave patients progressively disabled, and there are no treatments that can completely halt its progress forever.
However, scientists are unsure as to the precise reasons for the errant immune system response.
The Norwegian researchers found that the risk was increased for smokers regardless of whether they had quit by the time symptoms appeared, or were still smoking.
They looked at 22,312 people aged between 40 and 47, asking them about their smoking habits, and whether they had been diagnosed with MS.
MS remains relatively rare - from this group, there were 87 people with the condition.
However, men who smoked were 2.75 times more likely to have been diagnosed with MS, women smokers 1.61 times more likely.
This does not mean that - even for a smoker - the risk of developing MS becomes high.
The disease generally emerged 15 years on average after the start of smoking.
Dr Trond Riise, who led the study, said: "This is one more reason for young people to avoid smoking.
"Hopefully, these results will help us learn more about what causes MS by looking at how smoking affects the onset of the disease."
Smoking has already been linked to other "autoimmune" disorders, in which the immune system launches attacks on the body.
And it has already been conclusively associated with an increased risk of lung cancer, heart disease and stroke.
Dr Gary Franklin, a neurologist at the University of Washington School of Public Health and Community Medicine in Seattle, said that it was likely that smoking might help bring on the disease in someone whose genetic makeup made them vulnerable.
He said: "Neither having the susceptible genes by themselves, nor being exposed to the environmental factors alone, is enough to cause the disease."
A spokeswoman for the anti-tobacco group Action on Smoking and Health (Ash)
said: "We know smoking increases
the risk of about 50 different diseases including lung cancer, heart disease,
stroke, emphysema, diabetes and TB. Now it appears to be linked to MS as well.
"Since MS is an autoimmune disease it suggests that an effect on the immune
system is involved, which is something else to worry about.
"There are probably
all sorts of other diseases smoking could be implicated in that we don't know
Mike O'Donovan, chief executive of the Multiple Sclerosis Society, said: "These are interesting findings which, as the researcher says, need further investigation.
"We must also remember that more than one factor, including a persons genetic make-up, is almost certainly involved in whether MS occurs."