Molecularly targeted, customized therapies are designed based on the molecular portraits of cancer tissue. The efficacy of targeted therapy in individual patients depends on the contribution of single individual cancer cells within the context of their microenvironment. We have developed an in vitro model of human mammary epithelial-stromal cocultures to answer specific clinical questions related to breast cancer, to provide a tool with which to identify a signature in each breast tumor, and to identify the metabolic molecular targets of therapy in an attempt to optimize the efficacy of targeted therapy in each patient. Fifty-five human breast cancer samples were obtained through surgery. Epithelial and stromal cells were isolated from tissue specimens by differential centrifugation, and cryopreserved. Western blot analysis and RT-PCR were used to identify the tissue-specific expression patterns of cancer cells. Dose-response curves were constructed for the aromatase inhibitor formestane and for herceptin, and a 3-(4,5-dimethylthiazol)-2,5-diphenyltetrazolium bromide assay was done for combined treatment. We collected and cryopreserved, for future use, viable living cells from 55 breast tumor specimens from which we derived short-term cocultures. The presence of cytokeratins and vimentin was evaluated in 20 samples, and pHER2/neu and aromatase were evaluated in 4 cocultures. Formestane and herceptin had a cumulative growth-inhibitory effect on cocultures expressing epidermal growth factor receptors and aromatase. The in vitro model of human mammary epithelial-stromal cocultures reported herein can be used to examine, and to store, a patient's tumor-derived, living cells that retain the characteristics of the mother-tissue and respond, in vitro, to therapy.