The majority of breast cancer patients who achieve an initial therapeutic response to the human epidermal growth factor receptor 2 (HER-2)–targeted antibody trastuzumab will show disease progression within 1 year. We previously reported the characterization of SKBR3-derived trastuzumab-resistant pools. In the current study, we show that HER-2 interacts with insulin-like growth factor-I receptor (IGF-IR) uniquely in these resistant cells and not in the parental trastuzumab-sensitive cells. The occurrence of cross talk between IGF-IR and HER-2 exclusively in resistant cells is evidenced by the IGF-I stimulation resulting in increased phosphorylation of HER-2 in resistant cells, but not in parental cells, and by the inhibition of IGF-IR tyrosine kinase activity leading to decreased HER-2 phosphorylation only in resistant cells. In addition, inhibition of IGF-IR tyrosine kinase activity by I-OMe-AG538 increased sensitivity of resistant cells to trastuzumab. HER-2/IGF-IR interaction was disrupted on exposure of resistant cells to the anti–IGF-IR antibody α-IR3 and, to a lesser extent, when exposed to the anti-HER-2 antibody pertuzumab. Heterodimer disruption by α-IR3 dramatically restored sensitivity to trastuzumab and resistant cells showed a slightly increased sensitivity to pertuzumab versus parental cells. Neither α-IR3 nor pertuzumab decreased HER-2 phosphorylation, suggesting that additional sources of phosphorylation other than IGF-IR exist when HER-2 and IGF-IR are not physically bound. Our data support a unique interaction between HER-2 and IGF-IR in trastuzumab-resistant cells such that cross talk occurs between IGF-IR and HER-2. These data suggest that the IGF-IR/HER-2 heterodimer contributes to trastuzumab resistance and justify the need for further studies examining this complex as a potential therapeutic target in breast cancers that have progressed while on trastuzumab.