Flaxseed (FS), an oilseed containing high amounts of the phytoestrogen lignan, secoisolariciresinol diglucoside (SDG), and n-3 fatty acid, alpha-linolenic acid-rich oil (FO), has been shown to inhibit the growth of established human breast tumors (MCF-7) in ovariectomized (OVX) athymic mice. However, the major FS component responsible for this effect and the mechanism(s) of its action are unclear. Hence, this study determined, in a 2 x 2 factorial design, the effect of SDG and FO, alone or in combination, on the growth of established human estrogen receptor positive (ER+) breast tumors and the potential mechanism(s) of its action. OVX mice with established ER+ human breast tumors (MCF-7) were treated for 8 wk with basal diet (BD, control) or BD supplemented with SDG (1 g/kg), FO (38.5 g/kg), or SDG + FO. All treatments reduced the tumor growth, but SDG had the greatest effect primarily through reducing tumor cell proliferation rather than increasing apoptosis. SDG had a main effect in the reduction of PS2, BCL2, and IGF-1R mRNA expression, whereas FO had a main effect only in PAKT reduction. SDG alone also lowered the ERalpha, ERbeta, EGFR, BCL2 mRNA, and PMAPK protein, indicating that its effect involves the modulation of the ER- and growth factor receptor-mediated signaling pathways.