A woman who takes the hormone estrogen after menopause in an effort to avoid heart disease and osteoporosis runs a slightly higher chance of developing breast cancer, according to a new study. The risk is greater among older women and does not appear to change with the addition of progesterone, a second sex hormone frequently prescribed with estrogen.

Published in today's New England Journal of Medicine, the research compared the experience of women who chose to take hormones to those who chose not to. It isn't known if these two groups were similar. For example, there is no way of knowing whether women with many cases of breast cancer in their families were as likely to take estrogen as women who had no relatives with the disease. Consequently, it is not known whether hormone use -- or something else -- explains the different rates of cancer.

Even with these heightened chances, however, breast cancer remains relatively rare. A 60-year-old woman, without a strong family history of breast cancer and not taking estrogen, has a 1.8 percent chance of developing the disease in five years, [Meir J.] Stampfer said. If the same woman has taken estrogen for five years and is continuing to take it, the risk of breast cancer rises to 3 percent.