The majority of breast cancer patients who achieve an initial therapeutic response to the HER2-targeted antibody trastuzumab will show disease progression within 1 year. Thus, the identification of novel agents that effectively inhibit survival of cancer cells that have progressed on trastuzumab is critical. In the current study, we show that the dual epidermal growth factor receptor (EGFR)/human EGFR-2 (HER2) kinase inhibitor lapatinib induces apoptosis in trastuzumab-resistant cells derived from the HER2-overexpressing SKBR3 breast cancer line. Lapatinib inhibited EGFR and HER2 signaling in resistant cells, blocking activation of downstream Akt, mitogen-activated protein kinase [corrected] Importantly, lapatinib also inhibited insulin-like growth factor I (IGF-I) signaling and growth-promoting effects in parental and resistant cells, and the cytotoxic effects of lapatinib were further enhanced by the IGF-I receptor-blocking antibody alphaIR3. As increased IGF-I receptor signaling has been implicated in trastuzumab resistance, our data strongly support further study of lapatinib as a potential therapeutic in breast cancers that have progressed on trastuzumab.