The epidermal growth factor receptor (EGFR) can be activated by several growth factors within the tumor microenvironment, and it can activate several signaling pathways. For tumor development, these EGFR-related signaling pathways may converge on several common nuclear transcription factors, one such transcription factor being STAT5. STAT5 plays an important role in the oncogenic signal transduction pathway in non-small cell lung cancer. In this study, we examined whether the epidermal growth factor (EGF) can stimulate cyclooxygenase-2 (COX-2) expression in human lung adeno-carcinoma A549 cells transfected with or without STAT5 siRNA or dominant-negative (DN)-STAT5, and identified the pathways involved in this response. We found that STAT5 siRNA significantly reduced EGF-induced COX-2 expression, and STAT5 phosphorylation. STAT5 phosphorylation predominantly mediates EGF-induced COX-2 promoter activity. STAT5 siRNA was found to inhibit COX-2 expression in resting A549 cells despite the absence of detectable activated phosphorylated STAT5. Using an adenoviral system, we expressed DN-STAT5 in human lung adenocarcinoma A549 cells in order to broaden the investigation and to determine the role of STAT5 in EGF-mediated COX-2 gene expression. The overexpression of DN-STAT5 significantly inhibited EGF-induced COX-2 expression, and we found that EGF induced the tyrosine phosphorylation of STAT5 and up- regulated COX-2 expression. DN-STAT5 also blocked COX-2 promoter activity. Our results demonstrate that EGF stimulates COX-2 expression in human lung adenocarcinoma A549 cells via the activation of the STAT5 pathway and that COX-2 expression may be independent of phosphorylated STAT5 in A549 cells in vitro.