Identification of genes/proteins that are differentially expressed in HER2-overexpressing breast carcinomas (BC) is essential in elucidating the mechanistic basis of their increased metastastic potential. With the goal of identifying a unique HER2-induced "cytokine signature" in BC, Human Cytokine Array III (RayBiotech, Inc.) simultaneously detecting 42 cytokines and growth factors on one membrane was used to determine the profile of cytokines in conditioned media obtained from MCF-7/Her2-18 cells, a MCF-7-derived clone engineered to stably express the full-length human HER2 cDNA, and from the MCF-7/neo control sub-line. We identified two inflammatory and proangiogenic CXC chemokines with at least a 10-fold increased expression in MCF-7/Her2-18 transfectants when compared with matched control MCF-7/neo cells: CXCL8 (IL-8; interleukin-8) and CXCL1 and (GRO; growth-related oncogene). HER2 up-regulation of IL-8 and GRO was validated by ELISA and further confirmed by switching off the HER2 signaling. Treatment with the tyrosine kinase inhibitor gefitinib (Iressa) returned the expression levels of IL-8 and GRO back to the baseline observed in HER2-negative MCF-7 BC cells. Moreover, IL-8 and GRO circulating levels were significantly higher in sera from HER2-positive BC patients. These findings reveal for the first time that (a) Enhanced synthesis and secretion of members of the IL-8/GRO chemokine family, which have recently been linked to estrogen receptor (ER) inaction, increased cell invasion, and angiogenesis, may represent a new pathway involved in the metastatic progression and endocrine resistance of HER2-overexpressing breast carcinomas; and (b) Circulating levels of IL-8 and GRO cytokines may represent novel biomarkers monitoring BC responses to endocrine treatments and/or HER2-targeted therapies.