So-called "good" cholesterol does not always live up to its reputation and can increase the risk of heart disease in some people, research has shown.
High density lipoprotein (HDL) has long been thought of as beneficial because of the way it transports harmful substances away from artery walls.
But a particular genetic mutation may cause the cardiovascular hero to go over to the dark side, scientists have discovered.
People with the defective SCARB1 gene were found to have high levels of HDL, but also an 80% greater chance of developing heart disease than those without the mutation.
The risk increase is almost the same as that caused by smoking.
Dr Adam Butterworth, from the British Heart Foundation-funded Cambridge University Cardiovascular Epidemiology Unit, said: " We found that people carrying a rare genetic mutation causing higher levels of the so-called 'good' HDL-cholesterol are, unexpectedly, at greater risk of heart disease.
"This discovery could lead to new drugs that improve the processing of HDL-C to prevent devastating heart attacks."
Coronary heart disease is the UK's biggest killer, responsible for nearly 70,000 deaths each year, almost all due to heart attacks.
Harmful cholesterol, low density lipoprotein (LDL), plays a central role in the build-up of fatty material on artery walls, restricting blood flow to the heart.
HDL is known to carry fatty molecules out of artery walls and transport them to the liver for disposal.
The international team of scientists looked at the DNA of 328 individuals with very high levels of HDL in their blood and compared it with samples from people with relatively low HDL levels.
Next the researchers examined the effect of the mutation on HDL and heart disease in a population of more than half a million.
Dr Butterworth said: "Large-scale collaborative research like this paves the way for further studies of rare mutations that might be significantly increasing people's risk of a deadly heart attack. These discoveries also give researchers the knowledge we need to develop better treatments."