Estrogen deprivation therapy with selective aromatase inhibitors (SAIs) has become the standard of care for postmenopausal women with tamoxifen-resistant advanced breast cancer. About 1/3 of patients benefit from this therapy. There is interest in treating endocrine therapy refractory breast cancer with the recombinant DNA-derived humanized monoclonal antibody trastuzumab. When given alone, trastuzumab has an endocrine therapy-like risk benefit ratio. Trastuzumab targets ErbB2 (HER2/neu). Some breast cancers may coexpress ER and ErbB2.