Aromatase inhibitors are effective and well tolerated drugs in the endocrine therapy of estrogen-receptor positive breast cancer in postmenopausal women. Questions remain about the long-term side effects and safety profile of aromatase inhibitors. The results of ongoing studies may indicate the role of aromatase inhibitors in the prevention of breast cancer. The effectiveness and safety of aromatase inhibitor monotherapy in premenopausal breast cancer patients is unknown, so this is an area of future exploration. Our results in the chemoprevention of mammary carcinogenesis in female rats pointed to the potential favourable effects of aromatase inhibitors--anastrozole and letrozole in premenopausal women affected by breast cancer.