Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors that are involved in normal cytokine and growth factor signaling. Recent studies have shown that certain STAT family members, most notably STAT3, are constitutively activated by various oncoproteins, such as Src. Furthermore, STAT3 signaling is constitutively activated with high frequency in diverse human tumors, including breast carcinoma, head and neck squamous cell carcinoma, multiple myeloma and various leukemias. These findings indicate that downstream target genes of STAT3 contribute to malignant progression. To date a small number of genes have been shown to be regulated by STAT3, including cylcin D1, p21WAF1, Bcl-x, Mcl-1 and c-Myc, which are important in cell cycle control and apoptosis. However, it is likely that additional STAT3-regulated genes participate in oncogenesis. Using Affymetrix microarray chip technology, we have defined a list of genes that are associated with STAT3 activation in cells transformed by the Src oncoprotein and in model human breast carcinoma cell lines. We identified altered gene expression attributed to STAT3 by examining multiple cell lines with different levels of STAT3 activity. The list was refined by altering the STAT3 activation status of various cell lines following several approaches, including the use of pharmacologic inhibitors of STAT3 signaling. We employed a range of statistical approaches to identify genes that most accurately correspond to STAT3 activity in the context of oncogenesis. We have defined a molecular fingerprint of candidate STAT3-regulated genes that potentially contribute to malignant progression of breast cancer.