Estrogen receptor (ER) has been a successful target for effective prevention and treatment strategies in breast cancer, whereas growth factors and their signaling molecules are proving to be effective treatment targets as well. Understanding the interaction between ER and growth factor signaling pathways should provide clues to optimal treatment approaches and new strategies to overcome and prevent endocrine resistance. Cross-talk between ER and signal transducer and activator of transcription 5 (Stat5) has also been reported. Stat5 regulates growth, differentiation, and survival of mammary and hematopoietic cells. The role of Stat5 in breast cancer has not been established, although Stat5 is critical for some hematopoietic malignancies. We have analyzed the role of Stat5 in the progression of ER-positive breast cancer cells such as T47D and MCF7 in which Stat5b is constitutively activated. Adenoviral-mediated dominant-negative Stat5 induced apoptosis in T47D cells but not in caspase-3 negative MCF7 cells. Our study indicates that targeting Stat5 may represent a new strategy to suppress estrogen receptor activity and induce apoptosis in Stat5-activated, ER-positive breast cancer.