Breast cancer is significantly less prevalent among Asian women, whose diets contain high intake of soy products and tea. The objective of our present study was to identify the combined effects of dietary soy phytochemicals and tea components on breast tumor progression in a clinically relevant in vivo model of MCF-7 androgen-dependent human breast tumor in female SCID mice. MCF-7 tumor growth, tumor cell proliferation and apoptosis, microvessel density, and expressions of tumor estrogen receptors were compared in mice treated with genistin-rich soy isoflavones (GSI), soy phytochemical concentrate (SPC), black tea (BT), green tea (GT), SPC/BT combination and SPC/GT combination. GSI and SPC led to dose-dependent inhibition of MCF-7 tumor growth via inhibition of cancer cell proliferation in vivo. GT showed more potent anti-breast tumor activity than BT. GT infusion at 1.5 g tealeaf/100 mL water produced significant (p < 0.05) reductions of 56% in final tumor weight. GT plus SPC at 0.1% of the diet further reduced final tumor weight by 72% (p < 0.005). Analysis of serum and tumor biomarkers showed that the combined effects of SPC and GT inhibited tumor angiogenesis, and reduced estrogen receptor (ER)-α and serum levels of insulin-like growth factor (IGF)-I. Our study suggests that dietary SPC plus GT may be used as a potential effective dietary regimen for inhibiting progression of estrogen-dependent breast cancer. © 2003 Wiley-Liss, Inc.