High levels of daily stress may lower a woman's risk of breast cancer, researchers believe.
There is confusion over the effect of stress on breast cancer risk
The findings contrast with past work suggesting stress doubles the risk.
The Danish team says it may be that regular doses of stress are good, while a short burst of acute stress, around a life event such as bereavement, is bad.
Their work in the British Medical Journal is based on survey responses of nearly 7,000 women living in Copenhagen between 1981 and 1983.
Experts said the research did not clear up whether or not stress is an important factor in breast cancer risk.
At the start of the study, the researchers asked the women what levels of stress they experienced routinely in their lives, and classified the results into low, medium and high levels.
Stress was defined as tension, nervousness, impatience, anxiety, or sleeplessness.
The researchers then tracked whether any of the women developed breast cancer over the next 18 years - 251 of them did.
The study authors worked out that women reporting high levels of stress were 40% less likely to develop breast cancer than women reporting low levels of stress.
For every increased level of stress on a six-level scale, women were 8% less likely to develop breast cancer.
Dr Naja Nielsen and colleagues suggest one explanation for the findings may be that sustained levels of high stress may affect levels of the female hormone oestrogen, which over time may have an influence on developing breast cancer.
More research needed
But they cautioned that this theory has not been tested, and research in this area so far has mainly been restricted to animals.
They also warned that stress-induced changes in hormonal balances are not a healthy response, and continued stress may play a damaging part in other illnesses, particularly heart disease.
Work published in the same medical journal in 2002 by a team from Cancer Research UK's London Psychosocial Group found emotional stress did not increase the chance that a breast tumour would return after treatment.
Dr Emma Pennery from Breast Cancer Care said: "We know from talking to women with breast cancer that some of them believe stress to be a contributory factor.
"This new study is therefore very interesting. However, studies have generally produced conflicting results in this area and excessive, prolonged levels of stress can contribute to other health problems and might also be difficult to avoid.
"This study serves as a reminder that we still know very little about the causes of breast cancer and it is likely several factors combine to increase an individual's risk.
"What we do know is that the risk increases with age and that for all women it is important to be breast aware throughout their lives."
Difficult to measure
Dr Sarah Rawlings of Breakthrough Breast Cancer said: "It's always hard to measure the impact of stress on breast cancer risk as it's difficult to untangle from other factors in our lives and everyone views their own stress levels differently.
"This study doesn't help us to draw further conclusions. However, maintaining a healthy balanced lifestyle is important - we know that high stress levels can lead to unhealthy behaviour, which may alter your risk of breast cancer and other diseases."
Pamela Goldberg, of Breast Cancer Campaign, said: "It is still unclear exactly how stress affects normal cells in the body.
"In addition to this, other known risk factors for breast cancer were not taken into account during the trial."
Dr Charmaine Griffiths, of the British Heart Foundation, said: "A degree of challenge is necessary for us to feel motivated and enthusiastic, but too much stress does damage your heart health.
"Getting the balance right helps us to lead a healthy, active lifestyle and cope with stress in a positive way.
"The message from the BHF is be aware of the warning signs and take action to manage your stress so that it does not damage your health."