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I found some info here:

How is breast feeding related to breast cancer?

http://www.cancerhelp.org.uk/help/default.asp?page=5105

Breast feeding can protect you against developing breast cancer. We don’t know exactly how breast feeding is protective but, after the publication of a large Cancer Research UK study in 2002, we know that it definitely is.

The study compared breast feeding history in women who had breast cancer with women who hadn’t. It was a very large study, involving the histories of 50,000 women with breast cancer and nearly 100,000 women without.

The longer the women had fed for during their lifetime, the less likely they were to get breast cancer. According to the researchers, this was a very striking finding. They made sure that the women’s age; menopausal status, ethnic origin, number of births and their age at the birth of their first child were all taken into account. Breast feeding still lowered breast cancer risk by 4.3% for every year of feeding. There is also a 7% reduction in risk of breast cancer for each child born.

A 4% lowering of risk doesn’t sound much but, as breast cancer is quite a common disease in developed countries, breast feeding every child for an extra 6 months would mean about 1,000 fewer cases of breast cancer in Britain each year.

This research is a major step in explaining the difference in breast cancer rates between the Western world and developing countries. In developing countries, women tend to have more children and to feed each of them for much longer. Interestingly, in Japan 90% of women who have children breast feed. Japan is often talked about in relation to the incidence of breast cancer because, although it is obviously a developed country, breast cancer rates are much lower than they are in Western countries. Usually, people talk about diet as the explanation for this. But it may well be cultural differences in feeding babies that explains it.

These findings are important for helping us to prevent future cases of breast cancer. But the research may also help us in developing treatments. Any new knowledge about how breast cancer is triggered can help scientists to develop treatments to tackle it.

Researchers are now looking into whether breast feeding can help to protect women who carry one of the breast cancer faulty genes - BRCA1 and BRCA2. One Swedish study, published in 2004, concluded that breast feeding may reduce breast cancer risk for BRCA1 carriers who breast fed for more than a year in total, but there was no difference for BRCA2 carriers. There are other studies and it isn’t possible to draw definite conclusions from just this one study.