Flaxseed is a rich source of dietary lignans. Experimental studies suggest lignans may exert breast cancer preventive effects through hormonal mechanisms. Our aim was to study the effects of flaxseed on serum sex hormones implicated in the development of breast cancer. Forty-eight postmenopausal women participated in a 12-wk preintervention-postintervention study. Participants consumed 7.5 g/day of ground flaxseed for the first 6 wk and 15.0 grams/day for an additional 6 wk. Nonsignificant declines were noted over the 12 wk (95% confidence intervals) for estradiol (pg/ml), estrone (pg/ml), and testosterone (pg/ml): -4.4 (-12.6 to 3.9), -3.3 (-7.7 to 1.2), -4.7 (-17.8 to 8.5), respectively. Changes tended to be more pronounced in overweight/obese women, particularly for estrone (-6.5, -11.9 to -1.2; P = .02). Our results suggest that dietary flaxseed may modestly lower serum levels of sex steroid hormones, especially in overweight/obese women.