People who carry the haemophilia gene, but who do not have the illness may enjoy protection from heart disease, say doctors.
Haemophiliacs have blood which does not clot properly
Haemophilia is a disease in which the patient is unable to make enough of a particular chemical which helps clot the blood.
If untreated, then patients can bleed uncontrollably from the slightest injury.
It is caused by a defective gene - but not everyone who carries the gene develops the illness.
The gene is found on the X sex chromosome - males have just one copy of this, and women have two.
This means that if men have a defective gene, they have no "backup", and will become haemophiliac.
Women, on the other hand, need defective copies on both of their X chromosomes before illness will strike.
But a woman with just one correct copy will still produce less of the vital clotting factor than someone with two good copies.
Scientists have believed for some time the likelihood of getting heart disease might be in part related to the ability of the blood to clot - the more "sticky" the blood, the more likely that heart disease would follow.
The research team, from Leiden University in the Netherlands, wanted to find out if having a reduced ability to clot blood might be a protective factor.
They tracked down the records of mothers of more than 1,000 men who had developed haemophilia.
All of these women must carry one copy of the haemophila gene.
They looked to see how many had died from heart disease related causes.
They found that overall mortality from this cause was down 22% compared with the general population - although, as might be expected there was an increase in the risk of a serious brain haemorrhage.
This relationship between reduced clotting ability and heart disease could help doctors work out ways to protect people most at risk from heart problems.
Professor Frits Rosendaal, who led the research, said: "The mild decrease in blood coagulation that haemophilia carriers have affects the risk of heart attacks.
"This finding reemphasises the role of clotting, and changes in clotting, in the development of myocardial infarction, which in the future may have implications for the treatment of the disease."
Dr Anne Goodeve, head of the Molecular Haematology Unit at the University of Sheffield, told BBC News Online carriers of the haemophilia gene could be unaware of their blood's reduced ability to clot.
"They will probably be completely without symptoms," she said.
"A normal individual makes 100% of their clotting factors, while on average a carrier makes 50%.
"Most people can get by with this."
She said: "We've known for some time that people who make too much clotting factor have an increased risk of heart disease."
The study is published in The Lancet.