Stress doubles a woman's risk of developing breast cancer, scientists have found.
By Caroline Ryan
BBC News Online health staff in Copenhagen
Researchers from Sweden interviewed 1,350 women in the late 1960s and asked if they had suffered stress over the previous five years.
They then followed the women for 24 years and found those who reported having experienced stress had a 5.3% risk of developing cancer.
Those who reported no stress had a 2.5% risk.
The researchers told the European Cancer Conference in Copenhagen their study showed a 'significant association', but said they could not quantify the amount of stress needed to increase breast cancer risk.
Other studies have looked at the link, but most have asked women with breast cancer to try to recall if they were stressed prior to diagnosis.
In this latest study, women aged between 38 and 60 were studied. They were interviewed and examined at the beginning of the study in 1968 or 1969.
Researchers asked them if they had been stressed for a month or more in the previous five years, if they had felt tension, fear, anxiety or problems sleeping connected with their family or with work.
Their responses were classed into six categories, from 'no stress experienced at all' to 'experienced mental stress constantly during the last five years'.
The women then had follow-up examinations after five, 12 and 24 years.
Of 456 women who had experienced stress, 24 (5.3%) had developed breast cancer. Of the 894 who had not, 23 (2.5%) developed cancer.
Most women were diagnosed at around the age of 60, so the researchers say stress does not appear to cause women to develop breast cancer earlier.
The findings took into account factors such as alcohol use, weight, family history of breast cancer, smoking and whether, and when, they had had children.
'A step forward'
Dr Osten Helgesson of the Sahlgrenska Academy in Gothenburg, told the conference his study's findings were more valid than others which looked back, where women's diagnosis could colour their memories.
"This study is prospective and is based on information that is unbiased with respect to the disease."
He added: "Ours is one of only a very small number of prospective studies, and although our findings are significant, more and larger prospective studies need to be done.
"Our work takes us one step forward to discovering whether stress really is a predictor of subsequent breast cancer."
Professor Angela Ramirez, is head of a Cancer Research UK programme looking at the psychological and social aspects of breast cancer at Guy's and St Thomas's Hospital London.
She has carried out research which found stress was not linked to relapses in women who had already had breast cancer.
Professor Ramirez told BBC News Online said the research was interesting, but should be treated with caution.
"It's not hard evidence that being upset increases your risk of getting breast cancer."
Professsor Ramirez said the researchers had focused on self-reported stress, rather than objective measures such as financial bankruptcy.
But she said: "It's a long-standing belief that there could be a link between stress and breast cancer.
"I could imagine that if someone's stressed, they may suppress their immune system, and may affect the hormonal balance in the breast. That may be what the link is."
Delyth Morgan, chief executive of Breakthrough Breast Cancer said:"A number of studies have investigated potential links between stress and breast cancer but the results have been inconsistent.
"While this research is interesting, its findings should be treated with caution due to the very small sample size."
She added: "Further research is needed before any direct association between stress and increased breast cancer risk is confirmed.
"Breast cancer is an extremely complex disease and it's difficult to pin-point one single risk factor such as a stressful period in your life directly with the disease developing."