OBJECTIVE: To examine the association between isoflavones, androgens, and dietary composition and the risk of breast cancer in Australian postmenopausal women. DESIGN: Eighteen women with recently diagnosed breast cancer before surgery and 20 controls were recruited over a 12-month period. Both cases and controls were similarly assessed for urinary isoflavones, serum and urinary sex steroids, and dietary intake. RESULTS: Women with breast cancer had lower 24-h urinary daidzein compared with controls (cases: 31 [95% CI: 4, 234] nmol/day; controls: 427 [95% CI: 4, 234] nmol/day; p = 0.03), and there was a trend to lower urinary genistein excretion (cases: 25 [95% CI: 5, 132] nmol/day; controls: 155 [95% CI: 43, 550] nmol/day; p = 0.08). Total testosterone was higher in women with breast cancer compared with controls (cases: 1.3 [95% CI: 1.1, 1.5] nmol/L; controls: 1.0 [95% CI: 0.8, 1.11 nmol/L; p = 0.05). No significant differences were found for serum sex hormone binding globulin, free androgen index, dehydroepiandrosterone sulphate, estradiol and progesterone, or in urinary androgen metabolites, or in dietary intake with regard to fat, carbohydrate, protein, or fiber consumption between cases and controls. CONCLUSIONS: This preliminary study is the first report of low urinary daidzein and genistein in postmenopausal women with breast cancer. These findings are in keeping with the increasing observational data demonstrating a protective effect from phytoestrogens on breast cancer risk.