Researchers in New Zealand and Australia say there could be a genetic reason some people stay awake, but unable to move or speak while under anaesthesia.
Professor Jamie Sleigh from Waikato hospital says about one in 1000 people who are given a general anaesthetic, remain aware of their surroundings.
Professor Sleigh says the experience can be traumatic and can range from remembering sounds or smells to being able to feel the pain of the operation as it happens.
He says awareness is normally the result of not enough anaesthtic being given, but in about 10% of cases, the patients appear to have been given a proper dose.
Researchers are now investigating whether some people could be genetically resistant to the drugs.
They are asking for volunteers who have experienced awareness under anaesthesia to come forward to be interviewed and give a saliva sample.