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It has long been thought that multiple sclerosis isn't an equal-opportunity disease when it comes to afflicting women and men.

In recent decades, far more women have been getting the ailment, with most experts believing that multiple sclerosis in Canada afflicts about twice as many females as males.

But a team of researchers combing over Canadian data on those with multiple sclerosis has made a startling discovery: Women with the disease have inexplicably started to outnumber men by a ratio of more than 3 to 1.

And what is more, the researchers have found that the gender ratio, rather than being stable as has been commonly thought, has been rising in an almost uninterrupted fashion for at least 50 years, jumping from about 1.9 women for every man for those born in the 1930s, to 3.2 women for every man for those born around 1980.

The rise over this long period has been so pronounced that it has made multiple sclerosis an overwhelmingly female-dominated disease, even though it wasn't originally.

Until the 1940s most doctors believed there was no difference in the incident rate based on gender, and it was only in the 1950s and 1960s that physicians began to notice that the disease was more common in women.

A paper on the new findings is appearing in the November edition of the journal Lancet Neurology and the researchers who conducted the study of the Canadian multiple sclerosis data are speculating that something new has arisen in the environment in the past half-century to make women far more likely to develop the disease, a factor that inexplicably doesn't seem to affect men.

If multiple sclerosis rates are rising because of an environmentally induced cause among women, the discovery of what this might be would be highly significant because it suggests a possible avenue for preventing the often disabling disease.

"What is going on here is something presumably that is preventable," said George Ebers, a professor in the department of clinical neurology at the University of Oxford in the United Kingdom and lead author of the study. "We just need to find out what it is in the environment because it has to be environmental. Your genes don't change over two generations, three generations."

Scientists do not know what causes multiple sclerosis, although the disease is more prevalent in wealthy countries. Canada has one of the highest rates in the world, as do many nations in Northern Europe, which has led to speculation the cause may have something to do with vitamin D. Levels of the vitamin fall off in northern countries during the winter months because sunlight isn't intense enough to allow people to make it in their skin.

For those with multiple sclerosis, the research confirms what has often been the subject of anecdotal observations.

Marla Zaichick, who was diagnosed with the illness 10 years ago and is part of an organization called Women against MS, says the gender difference is readily apparent in groups of people with the disease. "You definitely see it," said Ms. Zaichick, who lives in Mississauga. "Yes, you do see men," but women are "just so much more predominant."

For Ms. Zaichick, who has a sister with MS and a brother who doesn't have it, one of the worries about the greater odds of women getting the disease is that she has a 10-year-old daughter, Ariel, and is concerned that she may be at higher risk. The paper said "the factors causing the increasing number of women with multiple sclerosis can only be speculated on, but given the short duration over which this is occurring, genetic change can be excluded."

Among the possible factors that could be causing the trend, the paper cited the changing role of women in the work force, dietary habits, changes in the timing of childbearing years, the falling age at which girls have their first period, and changes in outdoor activity levels.

If there is a connection to vitamin D, it could be through less time spent outdoors and an increase in time spent indoors at work. Prof. Ebers said a link to vitamin D is "certainly very plausible."

Prof. Ebers said previous research on half-siblings has found that mothers seem to be the ones influencing whether their children get MS, suggesting that something is increasing the risk of the disease during fetal life.

The rise in the sex ratio began before women started using oral contraceptives and before woman began smoking in great numbers.

About 1,000 new cases of the disease will be diagnosed this year in Canada, and an estimated 55,000 to 75,000 people are currently living with it. Lifetime medical costs per person with the disease are estimated at about $1.6-million.

Most cases are diagnosed between the ages of 15 to 40, making it a disease with large potential economic impact because it generally strikes people in the prime of their adult years.

The disease attacks the protective myelin covering of the central nervous system causing symptoms such as extreme fatigue, loss of balance, speech problems and even complete paralysis. Multiple sclerosis is considered an autoimmune disease, but it is not known why the body's immune system begins to malfunction and attacks myelin.

Previous research has made some odd discoveries about multiple sclerosis, such as a recent finding that those born in the Northern Hemisphere in May are more likely to develop it than those born in November, who had the lowest risk. This finding also suggests a role for vitamin D -- but in mothers -- because their May-born children would have less of the nutrient during fetal development, most of which took place in fall and winter.

By looking at a Canadian databank that has detailed information on about 27,000 people with the disease, Prof. Ebers and the other researchers calculated sex ratios starting with those born in 1931 up to 1980, or people who would have a relatively recent diagnosis. There were about 19,500 women and 7,600 men in the database.

Because of the large number of people whose information is in the database, one of the most extensive in the world, the results of the research are considered statistically significant and not due to chance.