Background: The mammalian lignans enterolactone and enterodiol are produced in the colon by the action of bacteria on the plant precursor secoisolariciresinol diglycoside, which is found in high concentrations in flaxseed. Objective: Two experiments were conducted to determine 1) whether there is a dose response in urinary lignan excretion with increasing flaxseed intake, 2) whether flaxseed processing affects lignan excretion, 3) peak plasma lignan concentrations, and 4) plasma lignan concentrations after chronic supplementation. Design: Nine healthy young women supplemented their diets with 5, 15, or 25 g raw or 25 g processed (muffin or bread) flaxseed for 7 d during the follicular phase of their menstrual cycles. Twenty-four-hour urine samples were collected at baseline and on the final day of supplementation. As an adjunct to the 25-g-flaxseed arm, subjects consumed the supplement for an additional day and blood and urine samples were collected at specific intervals. All blood and urine samples were analyzed for enterolactone and enterodiol by gas chromatography-mass spectroscopy. Results: A dose-dependent urinary lignan response to raw flaxseed was observed (r = 0.72, P less than or equal to 0.001). The processing of flaxseed as a muffin or bread did not affect the quantity of lignan excretion. Plasma lignan concentrations were greater (P less than or equal to 0.05) than baseline by 9 h after flaxseed ingestion (29.35 +/- 3.69 and 51.75 +/- 7.49 nmol/L, respectively). The total plasma area under the curve was higher on the eighth than on the first day (1840.15 +/- 343.02 and 1027.15 +/- 95.71 nmol.h/L, respectively). Conclusion: Mammalian lignan production from flaxseed precursors is dependent on time and dose but not on processing.