The declining levels for adults aged 20 and older occurred from the 1960s to 2002, fueled mostly by cholesterol reductions in older adults and thanks mostly to medication including statins, the researchers said.

Statins -- which include such widely used medicines as Lipitor, Zocor and Pravachol -- can dramatically reduce levels of LDL cholesterol, the bad kind that can clog arteries and lead to heart attacks. The drop in Americans' overall cholesterol levels resulted from a decline in LDL, according to the study.

The study appears in Wednesday's Journal of the American Medical Association. It is based on a comparison of data from periodic government health surveys.

Average Total Cholesterol Levels Dropped

Between 1960 and 2002, average total cholesterol levels for men and women ages 20 to 74 dropped from 222 milligrams per deciliter of blood to 203, mostly because of declines in people 50 and up. Among Americans ages 60 to 74, average levels fell from 232 to 204 in men (a 12 percent decline) and from 263 to 223 in women (down 15 percent). Below 200 is considered desirable for people at average risk for heart disease.

Also, in the study's final decade, the percentage of adults with high cholesterol -- reaching 240 -- fell from 20 percent to 17 percent, about eight years sooner than the government's goal of reaching the 17 percent mark by 2010, said study co-author Dr. James Cleeman, coordinator of the government's National Cholesterol Education Program.

At the same time, the portion of adults using cholesterol-lowering drugs, mostly statins, increased from 3.4 percent to 9.3 percent, with higher rates in the oldest Americans.

The study also contained an ominous sign -- a slight increase in levels of triglycerides, another blood fat linked to heart disease. The researchers said the increase -- if it is real and not a statistical fluke -- probably reflects rising obesity rates.

Other government data have shown that between 1988 and 2002, the percentage of overweight American adults climbed from 56 percent to 65 percent, while obesity rates increased from 23 percent to 30 percent. Obesity is often accompanied by high cholesterol levels, and both factors raise the risk of a heart attack or a stroke.

Physical Activity and Losing Weight

"A lot of people think once they've gone on statin drugs, they don't need to diet and exercise anymore," said Dr. Robert Eckel, president of the American Heart Association.

Statins can lower the risk of heart attacks but in some cases they may simply postpone such events, said Dr. Steven Nissen, a Cleveland Clinic cardiologist who was not involved in the study.

"Statins are great but if you put statins in the water supply, cardiovascular disease would still be the leading cause of death in America," Nissen said.

Average levels of HDL cholesterol, the good kind that helps remove fats from blood, remained mostly stable during the study. Researchers increasingly believe that boosting HDL levels -- and not just lowering LDL cholesterol -- is a key to preventing cardiovascular disease.

Physical activity and losing weight can boost HDL levels, and Eckel said lifestyle changes "need more attention."

Drug companies also are racing to create HDL-boosting medication.

Cardiovascular Disease Remains No. 1 Killer

A separate report this week from Quest Diagnostics, a leading provider of diagnostic testing, found more recent declines. Between 2001 and the end of 2004, average LDL levels fell from 123.7 to 111.7 in US adults 20 and older under a doctor's care. The largest declines were in adults 70 and older.

The report is based on nearly 80 million test results reported by Quest labs nationwide.

Clifford Johnson, a researcher with the Centers for Disease Control and Prevention and senior author of the JAMA study, called the government figures a glimmer of good news, although cholesterol levels were mostly unchanged in adults under 50.

Cleeman said a slight reduction in Americans' consumption of saturated fat probably contributed to the LDL cholesterol declines.

Annual deaths from heart disease in the United States dropped from nearly 800,000 in the late 1980s to about 650,000 in 2002. Cleeman said falling cholesterol levels may have contributed to that decline. Still, cardiovascular disease remains the nation's No. 1 killer.

Notable declines in LDL cholesterol levels were observed between 1976 and 2002, when average levels dropped from 138 to 123 in all adults. Again, the largest declines were in men over 60 and women over 50. Government guidelines set the upper limit at 130 for people at average risk for heart disease.