The numbers, reported Wednesday at an American Stroke Association conference in California, come from the first large nationwide study of stroke hospitalizations by age. Government researchers compared hospitalizations in 1994 and 1995 with ones in 2006 and 2007.

The sharpest increase — 51 percent — was among men 15 through 34. Strokes rose among women in this age group, too, but not as fast — 17 percent.

"It’s definitely alarming," said Dr. Ralph Sacco, American Heart Association president and a neurologist at the University of Miami. "We have worried for a while that the increased prevalence of obesity in children and young adults may take its toll in cardiovascular disease and stroke," and that appears to be happening, he said.

Stroke still takes its highest toll on older people. For those over 65, there were nearly 300 stroke cases among 10,000 hospitalizations in the more recent period studied. For males 15 to 34, there were about 15 stroke cases per 10,000, and for girls and women in that age group there were about 4 per 10,000.

Several small studies had recently suggested an ominous rise among the young and among middle-aged women.

"We were interested in whether we could pick that up in a much larger, nationwide dataset," said Dr. Mary George, a stroke researcher at the U.S. Centers for Disease Control and Prevention.

They examined federal records from a sample of hospitals in 41 states, covering about 8 million cases each year. They looked at the percentage of all hospitalizations for stroke by gender and in six age groups.

For every 10,000 hospitalizations in 1994-95 compared with 2006-07, strokes rose:

—51 percent, from 9.8 to 14.8, among males 15 to 34 years old

—17 percent, from 3.6 to 4.2, in females 15 to 34

—47 percent, from 36 to 52.9, in males 35 to 44

—36 percent, from 21.9 to 30, in females 35 to 44

"The increases seen in children are very modest, but they are more so in the young adult age groups, and we feel that deserves further study," George said.

Better awareness of stroke symptoms and better imaging methods for detecting strokes in young people could account for some of that change, but there is no way to know, she said.

Trends went the opposite way in older people. Strokes dropped 25 percent among men 65 and older (from 404 to 303 per 10,000 hospitalizations), and 28 percent among women in this age group (from 379 to 274). Doctors think better prevention and treatment of risk factors such as high blood pressure in older people may be contributing to the decline.

At the University of California at Los Angeles, doctors are seeing more strokes related to high blood pressure and clogged arteries in younger people, said Dr. Jeffrey Saver, director of the stroke center at UCLA.

Early estimates from 2007 death certificates suggest that stroke is now the nation’s fourth leading cause of death instead of the third, partly because of better treatments and prevention among the elderly. "But at the same time we’re seeing this worrisome rise in mid-life," Saver said.

Allison Hooker, a nurse who coordinates stroke care at Forsyth Medical Center in Winston-Salem, N.C., said her hospital also is seeing more strokes in younger people with risk factors such as smoking, obesity, high blood pressure, alcohol overuse and diabetes.

"I’d say at least half of our population (of stroke patients) is in their 40s or early 50s," she said, "and devastating strokes, too."

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Online:

Stroke conference: www.strokeconference.org