We have recently demonstrated that a natural dietary supplement BreastDefend (BD), which contains extracts from medicinal mushrooms (Coriolus versicolor, Ganoderma lucidum, Phellinus linteus), medicinal herbs (Scutellaria barbata, Astragalus membranaceus, Curcuma longa), and purified biologically active nutritional compounds (diindolylmethane and quercetin), inhibits proliferation and metastatic behavior of MDA-MB-231 invasive human breast cancer cells in vitro. In the present study, we evaluated whether BD suppresses growth and breast-to lung cancer metastasis in an orthotopic model of human breast cancer cells implanted in mice. Oral application of BD (100 mg/kg of body weight for 4 weeks) by intragastric gavage did not affect body weight or activity of liver enzymes and did not show any sign of toxicity in liver, spleen, kidney, lung and heart tissues in mice. Moreover, BD significantly decreased the change in tumor volume over time compared to the control group (p=0.002). BD treatment also markedly decreased the incidence of breast-to-lung cancer metastasis from 67% (control) to 20% (BD) (p<0.05) and the number of metastases from 2.8 (0.0, 48.0) in the control group to 0.0 (0.0, 14.2) in the BD treatment group (p<0.05). Finally, anti-metastatic activity of BD in vivo was further confirmed by the downregulation of expression of PLAU (urokinase plasminogen activator, uPA) and CXCR4 (C-X-C chemokine receptor-4) genes in breast tumors. In conclusion, BD may be considered as a biological therapeutic agent against invasive breast cancers.