Phytoestrogens are defined as plant substances that are structurally or functionally similar to estradiol. We report the associations of two major phytoestrogens, genistein and enterolactone, with breast cancer risk, using urinary specimens collected 1-9 years before breast cancer was diagnosed. The subjects were 88 breast cancer cases and 268 controls, selected from a cohort of postmenopausal women (n = 14,697) who participated in a breast cancer screening program. Mean levels of urinary genistein and enterolactone were determined by time resolved fluoroimmunoassay, using an average of two overnight urinary samples obtained from each participant on the first and the second screening rounds with a time interval of approximately 1 year. Odds ratios (ORs) of the highest to the lowest tertile of urinary phytoestrogen/creatinine concentrations and 95% confidence intervals (CIs) were computed. Higher urinary genistein excretion was weakly and nonsignificantly associated with a reduced breast cancer risk. OR for the highest tertile compared with lowest tertile was 0.83; 95% CI, 0.46-1.51. Higher urinary enterolactone excretion was weakly and nonsignificantly associated with an increased breast cancer risk. OR for the highest tertile compared with the lowest tertile was 1.43; 95% CI, 0.79-2.59. Tests for trends for both phytoestrogens were nonsignificant. We were not able to detect the previously reported protective effects of genistein and enterolactone on breast cancer risk in our postmenopausal population of Dutch women. Such an effect may be smaller than expected and/or limited to specific subgroups of the population.