Scientists have produced data which suggests smoking increases the risk of breast cancer.
Smoking is linked to many cancers
Previous research has produced mixed results - some has even suggested smoking may have a protective effect.
But a major study of more than 116,000 women by the California Department of Health Services suggests that smoking does pose a significant threat.
The work, which has been challenged by experts, is published in the Journal of the National Cancer Institute.
During the four years of the study, 2,005 of the women were diagnosed with invasive breast cancer.
The rate among women who were current smokers was around 30% higher than among those who had never smoked.
Women who started smoking before the age of 20, and those who started at least five years before their first pregnancy appeared to be most at risk.
Breastfeeding is known to help protect against breast cancer, but it might be that exposure to tobacco smoke may undermine this effect.
Heavy smoking or smoking over a long period of time also increased the risk.
However, there was good news for those who had kicked the habit. The researchers found no evidence of a significantly increased risk among former smokers.
There was also no evidence that passive smoking increased the risk
of developing cancer.
More work needed
The researchers said more work was required to investigate why smoking might increase the risk of breast cancer.
It is possible that toxins produced by tobacco smoke are stored in the fatty tissues of the breast.
However, they say: "Exposures to tobacco smoke, if causally related
to breast cancer, could offer one of the few available modifiable avenues for
preventing this disease."
Tobacco is already well known to cause other cancers, most notably lung
cancer, as well as other medical problems such as heart disease.
Amanda Sandford, of the anti-smoking charity Action on Smoking and Health, told BBC News Online that it had been harder to prove a link between smoking and breast cancer, than with other cancers.
But she said: "As this is such a big study it does seem that we can now say with some degree of confidence that breast cancer can be added to the ever growing list of serious diseases that can be caused by smoking.
"No organ in the human body is immune to the effects of tobacco smoke, and so it is plausible that cancer could strike anywhere."
Professor Valerie Beral, of Cancer Research UK, said an analysis by the charity of the worldwide evidence found smoking had little or no independent effect on the risk of breast cancer.
She said: "Smokers tend to drink more alcohol than non-smokers, the risk of breast cancer is clearly related to alcohol consumption, and the analyses of the association between breast cancer and smoking among women who drink are inextricably confounded by the effect of alcohol.
"Analyses restricted to non-drinkers avoid confounding by alcohol and show no increase in the risk of breast cancer among smokers."
Delyth Morgan, breast cancer charity Breakthrough Breast Cancer, said: "A number of studies have investigated the potential link
between smoking and breast cancer with conflicting results and more research is
needed to clarify a direct association between the two.
"Irrespective of any potential breast cancer risk, smoking is associated with lung and other cancers as well as heart disease and we would strongly advise all women and men not to smoke."