In patients with estrogen receptor (ER)-negative disease or ER+ hormone-resistant disease, the dominant influence on tumor cell growth is growth factors, e.g., epidermal growth factor (EGF), heregulins, and insulin-like growth factors acting through specific receptor tyrosine kinases at the cell surface. This superfamily of ligand-activated growth factor receptors triggers cascades of biochemical signals that influence tumor cell motility, invasiveness, angiogenesis, and survival, as well as proliferation. In breast tumors, expression of epidermal growth factor receptor (EGFR) and/or erbB2 is associated with poor prognosis; the therapeutic utility of blocking these receptors has been established using trastuzumab (Herceptin), a monoclonal antibody that blocks erbB2 signaling. An alternative therapeutic approach is offered by small molecule inhibitors of EGFR-TK, exemplified by ZD1839 (Iressa), a potent and selective EGFR-TK inhibitor. Resistance to tamoxifen is associated with up-regulation of the EGFR-TK pathway and mitogen-activated protein kinase activity is substantially increased in tamoxifen-resistant MCF-7 cells. ZD1839 treatment of tamoxifen-resistant MCF-7 cells blocks mitogen-activated protein kinase activity. Furthermore, treatment of wild-type MCF-7 cells with tamoxifen and ZD1839 prevents development of tamoxifen resistance. These data support the potential clinical utility of ZD1839 in tamoxifen-resistant breast cancer and suggest the possibility of preventing resistance by the early use of combination ZD1839 with antiestrogenic agents such as tamoxifen or ICI 182,780.