Vein blood clots happen when circulation is restricted in a deep vein -- often a leg. Deep vein thrombosis itself is not fatal but can kill if the clots move through the body to the lungs, causing a pulmonary embolism.
A pulmonary embolism is a sudden blockage in a lung artery, usually due to a blood clot that traveled to the lung from a vein in the leg. A clot that forms in one part of the body and travels in the bloodstream to another part of the body is called an embolus.
The team led by Henrik Sorensen, an epidemiologist at Denmark's Aarhus Hospital. said they did not know why vein clots, heart attacks and strokes were linked, but obesity felt could be a key factor.
The relative risk compared with people without blood clots in veins also remained about 20 to 40 percent higher for at least 20 years, said the study, published in the journal Lancet.
The researchers said the fact that vein clots seemed linked to strokes and heart attacks was surprising because the condition was very different from what had been considered the main cause of heart attacks.
It is known that heart attacks and strokes occur due to the effects of hardening artery walls. But blood clots do not cause veins to harden.
"The veins don't have the same process," Sorensen said. "Therefore it is surprising there is a link."
The team analysed data from Denmark's national medical databases over a 20-year period. They excluded patients with heart disease and then gauged the risk of heart attack and stroke in more than 25,000 patients with deep vein thrombosis.
Establishing a link between the conditions could help prevent heart attacks and stokes by getting more people on drugs aimed at reducing blood clots, Gordon Lowe, a researcher at Glasgow University, said in a commentary in Lancet.
The study is also important because many previous trials were far smaller and lacked statistical weight.
"Such studies have reported conflicting results, partly because of small event numbers and hence limited statistical power," he wrote.