For years, doctors have been saying that to prevent heart disease, patients should pay attention to both the so-called bad cholesterol, or LDL, and the good cholesterol, or HDL. The good, they said, can counteract the bad.

But now, some scientists say, new research has called into question whether high levels of the good cholesterol are always good and, when they are beneficial, how much.

While some heart experts are not ready to change their treatment advice, others have concluded that HDL should play at most a minor role in deciding whether to prescribe cholesterol-lowering drugs. In the meantime, doctors are calling researchers and asking what to do about patients with high HDL levels, or what to do when their own HDL levels are high, and patients are left with conflicting advice.

"There is so much confusion about this that it is unbelievable," said Dr. Steven Nissen, a cardiologist at the Cleveland Clinic.

The good cholesterol hypothesis comes from studies like the Framingham Heart Study that followed thousands of people for years to see who developed heart disease. The studies showed that if two people had the same levels of the bad cholesterol, LDL, but different levels of the good cholesterol, HDL, the one with more HDL was less likely to have heart disease.

Researchers examining the biochemistry of the two molecules learned that they have opposite roles. Both transport cholesterol, the fatty substance used to make cell membranes and some hormones, but they carry it in opposite directions.

LDL ferries cholesterol to coronary arteries, where it imbeds and participates in the growth of plaque. HDL trucks cholesterol away from arteries, taking it to the liver where it is disposed of.

So with epidemiological studies showing reduced heart disease risk and science showing why, it would seem the picture was clear: the more HDL the better. One HDL molecule might even cancel one of LDL.

Too simplistic, says Dr. Daniel Rader, a cholesterol researcher at the University of Pennsylvania School of Medicine. "Yes, high HDL is generally a good thing, but it doesn't mean it is so powerful that it creates a total immunity to heart disease," he said.

Rader and others say, for example, that there are people that have high levels of HDL, but the HDL does not function properly. Instead of being protected from heart disease, these patients may be particularly vulnerable. A simple HDL measurement does not reveal whether a person's high level is good or bad.

"I really don't feel that treatment for high LDL should be withheld just because the HDL level is high," Rader added.