ATLANTA — 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.

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 data was available.