For decades, heart disease death rates have been falling. But a new study shows a troubling turn -- more women under 45 are dying of heart disease due to clogged arteries, and the death rate for men that age has leveled off.

Heart experts aren't sure what went wrong, but they think increasing rates of obesity and other risk factors are to blame.

The rates will have to be monitored to see if this is the beginning of a real trend. But if the data holds, the new study may be an early glimpse of the impact of escalating obesity and diabetes on U.S. deaths, said Wayne Rosamond, a University of North Carolina epidemiology professor and expert on heart disease statistics.

''This could be a harbinger of things to come,'' Rosamond said.

To be sure, the overall trend is still positive: From 1980 through 2002, the death rate from blocked heart arteries was cut in half for men and women over 35. Improvements in treatment and preventive measures, including cholesterol-lowering medications, get the credit.

But what's going on with younger adults is startling, said Dr. Anthony DeMaria, editor of the Journal of the American College of Cardiology, which is publishing the study and released it Monday.

''We have a pretty rosy view of how things are going in the war against cardiovascular disease,''DeMaria said. ''I view this paper as a wake-up call that says there is a very important segment of our population that needs some attention.''

Heart disease is the leading cause of death in the United States, killing almost 700,000 Americans each year.

Nearly 500,000 of those deaths are attributed to coronary heart disease, in which fat and plaque clog the arteries feeding blood to the heart, sometimes called hardening of the arteries. Heart attacks are a common result.

It can take many years for arteries to get dangerously blocked. About 93 percent of deaths occur in people 55 and older.

But a combination of factors -- including genetics, obesity and high cholesterol -- are sometimes fatal for younger adults. In 2002, about 25,000 men and 8,000 women ages 35 to 54 died of coronary heart disease.

The study was done by researchers at the U.S. Centers for Disease Control and Control and Prevention and Britain's University of Liverpool. They looked at U.S. vital statistics for artery-related deaths in adults ages 35 and older for the years 1980 through 2002, the most recent year for which data was available when the analysis was done.

When they compared age groups, they detected the worrisome difference. The study found the death rate for women ages 35 to 44 rose from 1997 to 2002, when the rate was 8.2 per 100,000 women, the highest it's been since 1987.

In actual numbers, the increase amounts to roughly 100 added deaths a year of women in that age group. That's a relatively small impact in the entire U.S. population.

Still, the results are statistically significant and a legitimate cause for concern, said Dr. Wayne Giles, director of the CDC's division of adult and community health.

''That's like an MD-88 crashing every year,'' he said, referring to a medium-size commuter jet plane.

The rates for men age 35 to 44 were relatively stable in the last few years of the study period. The rate was 26 deaths per 100,000 men in that age group in 2002.

The fact the male rate didn't worsen may indicate doctors are more likely to suspect heart disease in men that age than in women, said the CDC's Dr. Earl Ford, a study co-author.

For all ages, the female death rate fell to 261 from 514 per 100,000; the male rate fell to 430 from 898 per 100,000.