Women who are told their breast biopsies are negative for cancer are understandably relieved. Yet they may be worried about the future, since even benign breast disease is a known risk factor for later developing breast cancer.

A new study more carefully lays out what, exactly, that risk is for different types of benign breast disease and suggests that a large group of women may have less to worry about than previously thought.

The study, published this week in the New England Journal of Medicine, examined the medical histories of 9,087 women who had a biopsy of a benign breast growth between 1967 and 1991 at the Mayo Clinic in Rochester, Minn., whose researchers led the review. Typically they followed the women for 15 years to see if they later developed breast cancer, and indeed, 707 of them did. That reaffirms the notion that having benign breast disease increases the risk of later developing cancer. Yet the study's authors broke down that large group of women into smaller categories: those who have what are called nonproliferative lesions, those with proliferative lesions, and those with atypical hyperplasia. Those medical terms refer to how the breast cells appear under a microscope and whether they are actively growing.

Most women–almost 67 percent–in the group had the nonproliferative type of breast disease, and for them, there is good news: Unless they also had a strong family history of breast cancer (defined as one close relative with the disease before age 50 or two or more relatives with breast cancer, one of them close), their risk of developing the disease was no greater than usual. "That's reassuring for them," says Lynn Hartmann, the study's lead investigator, an oncologist at the Mayo Clinic. The women who had this type of benign growth and also a strong family history of breast cancer had a 62 percent higher than normal risk of developing breast cancer.

The other types of benign disease represented a greater risk of later having breast cancer: an 88 percent higher risk for those with proliferative disease and a 324 percent increase in the risk for the small group with atypical hyperplasia (this risk wasn't compounded by having a family history of breast cancer). No matter the type of disease, the younger the patients, the more likely they were to eventually develop breast cancer.

Those numbers on atypical hyperplasia sound scary, and indeed, Hartmann says women who fall into that category may be candidates for preventive medications like tamoxifen or to take part in trials studying new drugs that may stave off cancer. Yet, as an editorial accompanying the study notes, while the risk is 324 percent higher than expected, the expected risk is pretty low. About 5 in 100 women in the general population are expected to develop breast cancer if they're followed for an average of 15 years; among women with atypical hyperplasia, 19 will most likely develop breast cancer. (The number of women who will develop breast cancer over an entire lifetime isn't as easy to come by, since it involves following groups of women for many, many years.)

The bottom line, if you are one of the many women who will have a breast biopsy at some point (20 percent have one within a decade of beginning annual screening): If, as in most biopsies, no cancer is found, make sure you ask your doctor whether there was any atypia and, if so, whether you have options for preventive drugs. And know that scientists are now working to further identify which women are most at risk and which therapies might best help them.