More than 40% of heart attacks may go unrecognised by sufferers, researchers have suggested.
Heart attack victims may think they have severe flu
In a study of more than 4,000 people, Dutch scientists discovered a third of male heart attacks, and more than half of female, were not spotted.
They believe women may be less likely to suspect an attack as their symptoms tend to be less typical, for example they may have shoulder not chest pain.
Details of the study are published in the European Heart Journal.
But according to UK experts, monitoring has improved since the research was conducted in the 1990s.
The researchers, from Rotterdam's Erasmus Medical Centre, focused on people who took part in a long-term study on chronic disease.
All were healthy when they signed up to the study, and all had at least two ECGs (electrocardiograms) during its course.
The researchers examined this group for signs of heart attacks which may not have been picked up.
Overall they calculated that nine heart attacks took place for every 1,000 years of life among the study group.
They found that at all ages between 55 and 80 heart attacks were more likely to be spotted in men than women.
Researcher Dr Eric Boersma said it was likely that most heart attacks that went unspotted did so because they did not produce typical symptoms.
He said there were likely to be many reasons why heart attacks - known technically as a myocardial infarction (MI) - were less likely to be spotted in women.
"Men and women experience chest pain in different ways.
"Women may sense shoulder pain instead of chest pain, they may think they have severe flu that is taken a long time to recover from, and those with an inferior-wall infarction may complain of stomach pain.
"So women may hold back from reporting symptoms and doctors may also be in doubt whether or not to consider heart disease as a source of the complaints.
"It is also a problem that women and their doctors have traditionally worried more about death from breast and gynaecological cancer, than from heart disease."
Dr Boersma said that although the study was conducted in the Netherlands the results were likely to be equally applicable to any other developed country.
Professor Peter Weissberg, medical director at the British Heart Foundation, said: "Doctors have known for some time that certain patients, particularly the elderly and those with diabetes, can suffer a heart attack without knowing it.
"The Rotterdam study, which relied solely on ECGs, suggests that unrecognised heart attacks may be more common than was once thought, particularly in women.
"However, this study was carried out in the 1990s - today we are much more vigilant in identifying patients at risk of developing heart disease and treating them."
Professor Weissberg said regular ECGs could help identify patients who have unrecognised heart disease, and who might benefit from treatment.
"Still more needs to be done to raise awareness of the signs of a heart attack.
"Knowing the symptoms and acting quickly could be the difference between life and death."