OBJECTIVE: Phytoestrogens are popular in treatment of menopause, although scientific evidence is insufficient as to their efficacy. We studied the effects of daily use of isoflavonoids on climacteric symptoms and quality of life in patients with a history of breast cancer. METHODS: Sixty-two postmenopausal symptomatic women were randomized to use either phytoestrogen (tablets containing 114 mg of isoflavonoids) or a placebo for 3 months; the treatment regimens were reversed after a 2-month washout period. Fifty-six women completed the study. Menopausal symptoms were recorded on the Kupperman index and the visual analogue scale, and working capacity and mood changes were assessed via validated questionnaires. In addition, we followed the levels of phytoestrogens, follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol, and sex hormone-binding globulin. Liver enzymes and creatinine were also assessed at each visit. RESULTS: The phytoestrogen regimen raised the circulating levels of phytoestrogens (daidzein, genistein, equol) 19- to 106-fold. The Kupperman index was reduced by 4.2 +/- 9.6 (mean +/- standard deviation) (15.5%) during phytoestrogen use and similarly by 4.0 +/- 8.1 (14.7%) during placebo use (P nonsignificant). The quality of life parameters (working capacity, mood changes) were unaffected by phytoestrogen. In addition, the phytoestrogen regimen caused no changes in FSH, LH, estradiol, or sex hormone-binding globulin. Phytoestrogen treatment was well tolerated and caused no changes in liver enzymes, creatinine, body mass index, or blood pressure. Of the 56 women, 25 (44.6%) preferred the phytoestrogen regimen, 15 preferred the placebo (26.8%), and 16 (28.6%) reported no preference (nonsignificant). CONCLUSION: Pure isoflavonoids did not alleviate subjective menopausal symptoms in breast cancer patients.