Some premenopausal breast cancer patients use phytoestrogen-rich soy and flaxseed to alleviate side effects induced by drugs such as tamoxifen (TAM). Lignan-rich flaxseed protects against breast cancer and increases the effectiveness of TAM. This study determined the interactive effects of lignan-rich sesame seed (SS) and TAM on estrogen-responsive MCF-7 breast tumor growth and bone health in ovariectomized athymic mice under premenopausal-simulated conditions. Ovariectomized mice with an estrogen implant and established MCF-7 tumors were treated for 8 weeks as follows: (i) positive control fed basal diet (BD), (ii) SS group fed BD supplemented with 10% ground SS, (iii) TAM group with TAM implant fed BD, (iv) SS + TAM group with TAM implant fed BD supplemented with 10% SS, and (v) negative control fed BD with no estrogen implant. Palpable tumor data, adjusted for body weight, showed that SS does not inhibit MCF-7 tumor growth and tends to negate the tumor inhibitory effect of TAM by increasing cell proliferation and reducing apoptosis. SS alone and combined with TAM enhanced femur biomechanical strength but caused no differences in bone mineral content or bone mineral density in either the femur or lumbar vertebrae. SS is not protective and interacts adversely with TAM in MCF-7 breast tumors but induces beneficial effects on bone both alone and when combined with TAM.