Flaxseed (FS) at 10% level has previously been shown in short term studies (8 weeks) to increase the effectiveness of tamoxifen (TAM) in estrogen receptor positive human breast cancer. The objective of this study was to determine the longer term dose dependent effect of FS, alone and in combination with TAM, on tumor growth, the mechanism of action and the effect on bone health. Ovariectomized athymic mice were injected with estrogen receptor positive human breast cancer cells (MCF-7) and implanted with estrogen pellet to allow tumors to grow. When the tumors are established, the estrogen implant was removed to simulate the low plasma estrogen levels in postmenopausal women. The mice were then divided into groups and treated as follows: fed the basal diet (control), 5% FS (5FS), 10% FS (10FS), 5FS plus TAM implant, 10FS plus TAM implant. After 16-week treatment, tumor regrowth after initial regression was observed in the TAM group. 5FS and 10FS alone regressed the established tumors, similar to the control, and prevented the tumor regrowth induced by TAM. This effect was related in part to the down regulation of estrogen-related and signal transduction pathways. TAM treatment caused improvement in bone mass density and biomechanical strength. Although FS did not cause improvement on bone, it did not reduce the bone health benefits of TAM. In conclusion, after longer term treatment, FS dose dependently reduced tumor growth and prevented TAM-induced tumor regrowth through its effect on estrogen related and signal transaction pathways, without affecting the effect of TAM on bone.