HER2 gene amplification increases VEGF production in breast cancers; combined inhibition of HER2 and VEGF enhances response in xenograft models. The upregulation of VEGF in HER2-overexpressing breast cancers may contribute to the aggressive phenotype observed in HER2-positive breast cancer. New therapeutics targeting VEGF and/or its receptors may enhance the efficacy of trastuzumab monotherapy.