The objective of this study was to determine if 6 months of the aromatase inhibitor letrozole, administered to postmenopausal women taking a stable dose of hormone replacement remedy, would be safe and would modulate biomarkers of breast cancer risk. The intent was to reduce the proliferation marker Ki-67 while maintaining adequate systemic levels of estradiol so as to avoid perimenopausal symptoms. Postmenopausal women at high risk for development of breast cancer and taking a stable dose of estrogen or estrogen plus progestin were screened by random periareolar fine needle aspiration (RPFNA). To be eligible, the acquired breast epithelial cells had to be characterized as cytologic atypia or borderline atypia with > or =1,000 epithelial cells on the cytomorphology slide; plus > or =500 epithelial cells on a slide processed for Ki-67 immunocytochemistry. Forty-two women were enrolled in the one arm study and received 2.5 mg letrozole per day for 6 months, followed by repeat assessment of biomarkers. Ki-67 was reduced by a median relative value of 66%. There was no significant change in breast cell cytomorphology; ER weighted index score; serum estradiol, testosterone, or IGF-1:IGFBP-3 ratio; mammographic breast density, or frequency or severity of perimenopausal symptoms. Given the dramatic reduction in proliferation, the effect of letrozole on risk and response biomarkers should be explored further in a randomized, placebo-controlled Phase IIB breast cancer chemoprevention trial.