People under 40 are five times more likely to have a heart attack if they smoke, a study shows.
Campaigners say the study shows young smokers should quit
The international team of researchers said their findings put paid to the idea that only older smokers were at risk of heart disease.
Their findings, based on World Health Organization figures, are published in the journal Tobacco Control.
The British Heart Foundation said the report should act as a "profound warning" to young smokers.
The study was based on data from the World Health Organization international monitoring study of cardiovascular disease (MONICA) and risk factors. MONICA involved participants between the ages of 33 and 64 from 21 countries.
The researchers, from centres in Europe, China, Australia, New Zealand and North America, assessed all non-fatal episodes of heart disease occurring between 1985 and 1994.
The experts, who studied almost 23,000 non-fatal heart attacks in adults between 1985 and 1994, found that four-fifths of victims aged 35-39, were smokers.
Men aged between 35 and 39, who smoked, were almost five times as likely to have a non-fatal heart attack as their non-smoking peers.
The impact was even greater among women smokers in the same age band. They were over five times as likely to have a non-fatal heart attack.
Smoking accounted for almost two thirds (65%) of non-fatal heart attacks in men, and for over half (55%) in women aged between 35 and 39.
The risks for smokers in the age band 60 to 64 were lower, because of other contributory factors.
But the researchers found smoking still carried a higher risk for older women compared with men, which they said could be due to their being more sensitive to the effects of smoking.
Writing in Tobacco Control, the researchers, led by Dr Markku Mahonen of the KTL National Public Health Institute in Helsinki, Finland, said: "Although young people may acknowledge the well documented fact that cigarette smoking is a major risk factor for coronary heart disease, they, and even physicians, may think that this is only a concern in older age."
They added: "There is a particular need for public health programmes and anti-smoking campaigns targeted at young people to keep them healthy, and specifically from our results, to prevent the particular tragedy of heart attack at a young age."
Dr Tim Bowker, Associate Medical Director of the British Heart Foundation, said: "This should be a profound warning to younger smokers that they are not only damaging their health for later years and cutting their lives short, but are also putting themselves at a significantly higher risk of having a heart attack before they even reach middle age.
"This evidence should encourage young smokers to think about today, not just tomorrow."
Professor Andrew Peacock of the British Thoracic Society also urged young smokers to quit.
"The evidence of just how harmful cigarettes are continues to grow and grow.
"Everybody knows that smoking damages the lungs and this new study shows how it causes heart attacks in the young, but people need to be aware that there are more than 20 different ways that smoking can kill you even at a young age."