Acquired resistance to Herceptin is a major clinical problem in the treatment of HER2-overexpressing breast cancer. Understanding the molecular mechanisms leading to resistance will allow identification of novel therapeutic targets and predictors of therapeutic response. To this end, up-regulation of anti-apoptotic proteins has been associated with resistance to the HER2-targeted drug lapatinib, but has not yet been linked to Herceptin resistance. The aim of the current study was to determine if the Bcl-2 anti-apoptotic protein is a potential therapeutic target in cells with acquired Herceptin resistance. The BT474 HER2-overexpressing breast cancer cell line and BT474-derived acquired Herceptinresistant clones were used as models in this study. Bcl-2 and Bax expression were assessed by Western blotting. Proliferation assays were performed on cells treated with the Bcl-2 inhibitor ABT-737 in the absence or presence of Herceptin. Finally, the effect of PI3K inhibition or IKK inhibition on Bcl-2 expression and Herceptin sensitivity was examined by Western blotting and established proliferation assays. We show that cells with acquired resistance to Herceptin have an increased Bcl-2:Bax ratio. Resistant cells have increased sensitivity to ABT-737. Further, pharmacologic inhibition of Bcl-2 improved sensitivity to Herceptin in acquired resistant cells. Finally, PI3K and IKK inhibition downregulated Bcl-2 expression and increased sensitivity to Herceptin in resistant cells. Taken together, these new observations support further study of Bcl-2-targeted therapies in Herceptin-resistant breast cancers, and importantly, future investigation of Bcl-2 expression as a potential predictor of Herceptin response in patients with HER2-overexpressing breast cancer.