While smoking has been clearly established as increasing the risk of stroke, "there is not a lot of data out there on the actual dose response," said Dr. John Cole, the study's corresponding author and an assistant professor of neurology at the University of Maryland School of Medicine in Baltimore.

Cole and his colleagues interviewed 466 women who had had a stroke, and also 604 women who hadn't. All were between the ages of 15 and 49, and were either smokers, non-smokers or former smokers.

Any smoking at all doubles the risk of stroke, the study found. The risk was 2.2 times greater for women smoking one to 10 cigarettes a day, 4.3 times greater for those smoking 21 to 39 cigarettes a day, and 9.1 times greater for those smoking two packs a day or more, compared to nonsmokers.

The study also demonstrated the benefit of quitting smoking. Stroke risk declined as early as 30 days after a woman gave up smoking and returned to normal in about two years.

"Stopping is the best thing to do, but cutting back will also reduce the risk," Cole said.

Smoking raises the risk not only of stroke but also of heart disease by damaging blood vessels and making blood clots more likely, Cole said.

The study findings are published in the Aug. 15 issue of the journal Stroke.

"Cigarettes, among other tobacco products, are the only products that when used as directed are still guaranteed to do harm," said Dr. David A. Meyerson, director of cardiology consultative services at Johns Hopkins University Bayview Medical Center, and a spokesman for the American Heart Association.

"There are four major reasons why," Meyerson added. "Smoking disrupts the cells lining the blood vessels. It increases blood fibrogen levels, which makes blood more likely to clot. It increases the stickiness of platelets, the cells that form blood clots, and it also decreases the body's natural clot-dissolving mechanism."

And young women who might be unconcerned about smoking's link to stroke should also know that it causes premature aging, Meyerson said.

The new study is valuable "because of its size and its ethnic diversity," he said. "We see broadly how it applies to all young women."

About 20 percent of young American women are smokers, the report noted.

Cole said a similar study on young men is planned.

Another report in the same issue of the journal dealt with stroke and another subject of interest to young women -- and men as well: fat around the waist. A study of 1,137 German adults found that measures of "abdominal adiposity" were strongly associated with the risk of stroke and transient ischemic attacks, which are momentary stoppages of blood flow to the brain.

Waist fat was a better indicator of stroke risk than body mass index, a standard measure of obesity, said the report from neurologists at Saxon Hospital Arnsdorf. Better diet and more exercise were recommended as corrective measures.