Amplification of the human epidermal growth factor receptor 2 (HER2) gene occurs in 30% of breast cancers, and HER2 is an attractive target for the development of therapeutic drugs. Lapatinib, a dual tyrosine kinase inhibitor of HER2, is clinically active in patients with breast cancer positive for HER2 amplification. Although lapatinib improved the overall outcome for such patients, not all patients benefited from the treatment. Mutations in PIK3CA, which encodes the catalytic subunit of phosphatidylinositol 3-kinase (PI3K), have been identified in 8–40% of breast cancers. Although a positive correlation between HER2 overexpression and the presence of PIK3CA mutations has been described, the relation between the efficacy of lapatinib and such mutations has remained unclear. We have now investigated the effects of lapatinib in HER2 amplification-positive breast cancer cells with or without an activating PIK3CA mutation. We found that lapatinib-induced apoptosis in association with up-regulation of the proapoptotic protein BIM through inhibition of the MEK-ERK signaling pathway in breast cancer cells with HER2 amplification. RNA interference-mediated depletion of BIM inhibited lapatinib-induced apoptosis, implicating BIM induction in this process. The proapoptotic effect of lapatinib was less pronounced in cells with a PIK3CA mutation than in those without one. Lapatinib failed to inhibit AKT phosphorylation in PIK3CA mutant cells. Depletion of PIK3CA by si-RNA revealed that survivin expression is regulated by the PI3K pathway in these cells; furthermore, depletion of survivin by RNA interference or treatment with a PI3K inhibitor markedly increased the level of apoptosis in PIK3CA mutant cells treated with lapatinib. Our results thus suggest that inhibition of both PI3K-survivin and MEK-ERK-BIM pathways is required for effective induction of apoptosis in breast cancer cells with HER2 amplification.