Not all that long ago, a low cholesterol score was seen as a sign of relative good health and a low risk of heart disease.

But increasingly, doctors are identifying a group of people whose levels of L.D.L, the so-called bad cholesterol, are low, but who still appear to be at increased risk for atherosclerosis, heart attack and stroke.

They have a condition known as metabolic syndrome, a cluster of risk factors that include mild hypertension, elevated glucose levels, high triglycerides and low levels of H.D.L. cholesterol.

People with the syndrome also tend to have high levels of a protein, known as C-reactive protein, or CRP, which is released during inflammation and has recently been linked to heart disease.

"By far, the people we're seeing with heart disease are people with metabolic syndrome, because weight gain is the driving force and people are gaining weight," said Dr. Arshed Quyyumi, a professor of cardiology at the Emory School of Medicine.

Studies suggest that as many as 55 million Americans meet the diagnostic criteria for metabolic syndrome. But scientists still disagree about what causes the syndrome, how to diagnose it, how to treat patients who have it and even what it should be called.

In 1988, Dr. Gerald M. Reaven, an endocrinologist at Stanford, was the first to describe a cluster of low-level risk factors that tended to travel together and that substantially increased heart disease risk. He called it Syndrome X, and noted that the patients with the risk factors also had low levels of L.D.L., the type associated with higher heart disease risk.

"Everybody thought all heart disease came from having high L.D.L. cholesterol," Dr. Reaven said. "But no, a major group at risk for heart disease are people who are insulin-resistant and have these other factors, but who don't have high cholesterol."

Three years later, a report by the National Cholesterol Education Program at the National Institutes of Health recognized this constellation of risks, emphasized obesity as a central component, renamed it the metabolic syndrome (relegating Syndrome X to the history books) and provided diagnostic tests that would be easy for a primary care doctor to carry out.

The report advised that the syndrome should be diagnosed in people who had three of five risk factors: a large waist, high triglycerides, low H.D.L., somewhat elevated blood pressure and elevated fasting glucose.

But not everyone agrees. Some endocrinologists, including Dr. Reaven, worry that the diagnostic criteria exclude insulin-resistant people, whose cells are less sensitive to insulin, but who may not be overweight or have as many as three risk factors.

East Indians, for example, have a high rate of insulin resistance, though they tend to be thin. And people who are insulin-resistant, endocrinologists say, are also at high risk for heart disease and diabetes, as well as other illnesses like fatty liver disease (the leading cause of cirrhosis and end-stage liver disease), polycystic ovary syndrome and sleep-disordered breathing.

Many endocrinologists prefer a different name for the condition: insulin resistance syndrome.

"If there is any large public health value of looking at this clustering of risk factors, it is to help distinguish who among the obese have this clustering and are at the greatest risk for medical consequences," said Dr. Daniel Einhorn, an endocrinologist and director of the Scripps Whittier Institute for Diabetes in La Jolla, Calif.

The American Association of Clinical Endocrinologists has recommended that doctors widen the net by considering other factors, including body mass index; a more sensitive test of insulin resistance; age, exercise habits and ethnic background; and family history of Type 2 diabetes, hypertension and cardiovascular disease.

Some experts believe doctors should also look carefully for high levels of C-reactive protein because of its role in inflammation.

"CRP is one important part of the metabolic syndrome and an important cause of the consequences of metabolic syndrome," Dr. Einhorn said.