Epidemiologic, in vitro, and in vivo studies support the hypothesis that mammalian lignans have cancer protective effects. Flaxseed is the richest source of plant precursors to mammalian lignans. However, there are limited data on the lignans from processed foods containing flaxseed, despite increasing flaxseed use. Thus the objective of this study was to quantify the lignans in flaxseed-containing processed foods and their relationship to the percent flaxseed in the food. Twenty-five foods including raw flaxseed, homemade products containing flaxseed, and commercial breads and breakfast cereals with and without flaxseed were subjected to an in vitro fermentation designed to simulate the colonic environment necessary for the conversion of plant precursors to mammalian lignans. The lignan production from cereals containing flaxseed was significantly greater than that from their component grains (p < or = 0.01), and the lignan production from breads containing > 4% flaxseed was significantly greater than that from breads containing < 4% flaxseed (p < or = 0.05). Lignan production was significantly related to percent flaxseed in homemade products (r = 0.95, p < or = 0.01) and breakfast cereals (r = 0.997, p < or = 0.0001). Although a significant relationship was also observed in commercial breads (r = 0.58, p < or = 0.05), the correlation was lower because of the variability in different flaxseed varieties and the lignan contribution from other grains and oilseeds in these products. In conclusion, the addition of flaxseed to processed foods increased the production of mammalian lignans significantly. The amount of lignan production was linearly dependent on the percent flaxseed but was also influenced by other grains and the variety of flaxseed.