Men who do not openly express their anger if they are unfairly treated at work double their risk of a heart attack, Swedish research suggests.
Nearly 50 of the studied men died from a heart attack or heart disease
The researchers looked at 2,755 male employees in Stockholm who had not had a heart attack when the study began.
They were asked about how they coped with conflict at work, either with superiors or colleagues.
The researchers say their study shows a strong relationship between pent-up anger and heart disease.
Writing in the Journal of Epidemiology and Community Health, the researchers called the various strategies for keeping things bottled up, covert coping.
The men were asked what methods they adopted: whether they dealt with things head-on, whether they let things pass without saying anything, walked away from conflict, developed symptoms like headache or stomach ache or got into a bad temper at home.
They were checked for smoking, drinking, physical activity, education, diabetes, job demands and their freedom to take decisions.
Their blood pressure, body mass index and cholesterol levels were measured and they were aged 41 on average at the start of the study between 1992 and 1995.
Details of whether any of the men subsequently had a heart attack or died as a result of heart disease in the period up to 2003 were gathered from national registers of hospital treatment and deaths.
Up to 2003, 47 of the 2,755 men had a heart attack or died from heart disease.
The men who coped by sometimes or often walking away or who often let things pass without saying anything, had double the risk of a heart attack or dying from serious heart disease compared to men who challenged and dealt with the situation head-on.
Developing a headache or stomach ache or getting into a bad temper at home, did not increase the risk of heart attack or heart disease.
The researchers believe that anger can produce physiological tensions if it is not released and that these lead to increases in blood pressure which eventually damage the cardiovascular system.
Dr Constanze Leineweber, who led the study from the Stress Research Institute in Stockholm, said: "There has been research before pointing in this direction but the surprise is that the association between pent-up anger and heart disease was such a strong one.
"I think men can't help how they behave in conflict situations - it's not something they think about, it's just how they react instinctively.
"If you are smoking and don't exercise you would be much more conscious of the risk."
Judy O'Sullivan, senior cardiac nurse for the British Heart Foundation, said: "Stress itself is not a risk factor for heart and circulatory disease, but some people's responses to stress, such as smoking or overeating, can increase your risk.
"We all find different things stressful and symptoms of stress can vary, but the important thing is that we need to find ways of coping with it in our lives in a positive way, whether at work or home."