Health experts recommend keeping levels of the bad LDL cholesterol as low as possible and advise eating foods high in HDL cholesterol - such as olive oil and fatty fish.
The good cholesterol is generally associated with reduced heart risks, since it usually offsets the artery-clogging effects of the low density LDL form.
Now it seems that for some patients, high HDL levels, far from being helpful, can actually lead to heart attacks.
A rare genetic mutation causing the body to have high levels of HDL can, paradoxically ,lead to a higher heart risk - almost equivalent to smoking, scientists claim.
Continue reading after the cut.....
The study found people with a mutation in a gene called SCARB1, which affects one in 1,700 people, had very high levels of 'good' cholesterol but an 80 per cent greater risk of heart disease.
Most of the cholesterol circulating in our blood is made by the liver, mainly from saturated fats.
LDL transports cholesterol from the liver to cells where it is needed for such processes as strengthening cell walls and making hormones.
HDL does the opposite, taking surplus cholesterol from cells back to the liver, where it is recycled or removed from the body in bile.
Scientists found the mutation was preventing HDL from ferrying the fat it had collected in the liver for processing.
Dr Adam Butterworth, one of the researchers from the University of Cambridge, said the findings were 'unexpected'.
'This is significant because we had always believed that good cholesterol is associated with a lower risk of heart disease,' he told the BBC News website.
'This is one of the first studies to show that some people that have high levels of 'good' cholesterol actually have a higher risk of heart disease so it challenges our conventional wisdom about whether 'good' cholesterol is protecting people from heart disease or not.'
He said the discovery could lead to new drugs that improve the processing of HDL-C to prevent devastating heart attacks.
The finding could also help explain why drugs that boost HDL have so far failed to have the same impact as statins, which lower the bad cholesterol.
Drug companies have been ploughing money into treatments that raise HDL, thinking that high levels can reduce the incidence of strokes or heart attacks but, so far, efforts have been unsuccessful.
Earlier this year, professor Eliano Navarese, an Italian cardiologist and director of SIRIO Medicine, a network of experts reviewing medical research, casted doubt over its perceived benefits to health.
'The thinking that increasing HDL, which is widely advised by clinicians, could provide health benefits has been denied by a growing body of evidence,' he said.
Doctors can carry out a routine blood test to check both HDL and LDL levels.
Professor Navarese suggested that patients who suspect they have an abnormally high HDL level may need additional tests, such as on their coronary calcium levels to know how much build-up there is and to identify their level of risk for furred-up arteries and heart disease.
The latest study by an 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.'
Peter Weissberg, medical director at the British Heart Foundation which supported the study, said the new findings had shed light on a major puzzle and could open up new medical avenues in the longer term.
'These unexpected findings pave the way for further research into the SCARB1 pathway to identify new treatments to reduce heart attacks in the future,' he said.
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