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Phytoestrogens have received media attention as a form of breast cancer prevention. Although epidemiologic studies support this claim, there are no prospective clinical trials demonstrating such a protective effect. This ongoing clinical trial, supported by an AICR Postdoctoral Award, aims to evaluate the effect of a phytoestrogen supplement on the breast tissue of postmenopausal breast cancer survivors. Disease-free post-therapy postmenopausal women (n = 60) with in situ or early invasive (Stage 0-II) breast cancer will be randomly assigned to receive either isoflavone tablets (100 mg/d) or placebo for 1 y. Biopsies of the uninvolved breast will be examined for proliferative changes in response to phytoestrogens as well as immunohistochemical breast cancer biomarkers. Mammographic density was assessed at the time of biopsy to correlate with histological findings. As secondary end points, menopausal symptoms, vaginal epithelial changes, endometrial histology and serum steroid hormones are also being measured. We began to screen subjects in June 2001. Since then, 631 breast cancer patients have been screened through the Seattle Cancer Care Alliance, and we have received 53 additional self- or clinician referrals. A total of 467 were found to be ineligible, 52 refused participation and 15 have consented to participate so far. The primary reason for ineligibility at our institution is disease stage (76%). Of the women who stated their primary reason for refusal, the most common reasons have been invasiveness of the trial procedures (15%) and unwillingness to take phytoestrogen supplements (31%). Of the 10 women enrolled in the study, none have reported side effects attributable to the isoflavone tablets. One woman developed a small hematoma after one of her 3 biopsies and we received two complaints associated with the dressings used, but there have been no other biopsy-related complaints. To increase recruitment yield, a mechanism to see patients who receive their oncological care outside the sponsoring institution has been developed, and a community outreach and education campaign regarding phytoestrogens has begun. Mammographic density will be followed to see whether it can serve as a noninvasive end point for breast epithelial proliferation.