Consumption of the phytoestrogen lignans, structurally similar to estrogen, has been associated with alterations in gene expression and estrogen metabolism. Furthermore, lignan consumption, subsequent changes in metabolizing enzyme expression, and genetic variability in these enzymes may alter estrogen metabolism and modify disease risk. Therefore, we investigated the effect of flaxseed on hydroxyestrone metabolite excretion by catechol-O-methyltransferase (COMT) and cytochrome P450 1B1 (CYP1B1) genotype. We conducted an intervention among 132 healthy, postmenopausal women, ages 46 to 75 years. Participants consumed 10 g ground flaxseed daily for 7 consecutive days. Blood and urine samples were collected at baseline and after the 7-day intervention. COMT Val 158 Met and CYP1B1 Leu 432 Val genotypes were determined using PCR-RFLP methods. Urinary 2-hydroxyestrone (2OHE1) and 16A-hydroxyestrone (16OHE1) were quantified by ELISA assay. The effect of genotype on intervention-related changes in estrogen metabolites was assessed with the Kruskal-Wallis test. Compared with baseline levels, postintervention levels of urinary 2OHE1 (ng/mg creatinine; mean F SD, 16.1 F 10.6 versus 9.3 F 6.9, postintervention and baseline, respectively; P < 0.01) and 2OHE1/16OHE1 ratios (mean F SD, 2.73 F 1.47 versus 1.54 F 0.75, postintervention and baseline, respectively; P < 0.01) were significantly higher.