Background: ¹⁸F-Fluoroestradiol (FES) is an estrogen analogue that has been shown to be a promising biomarker in ER imaging of breast cancer. FES uptake correlates to ER expression, provides qualitative and quantitative assessment of multiple tumor sites simultaneously, and can predict response to endocrine therapies. Tumor heterogeneity is a known feature of metastatic breast cancer. Our work and others has shown that patients that have tumors with high FDG-PET SUV and low FES-PET SUV uptake have a poorer prognosis. Biopsies of metastatic disease may be done initially for diagnosis of metastatic disease, but are generally only performed in the setting of target identification for clinical trials. A change in ER or HER2 expression, however, can result in a change in therapy. FES-PET imaging offers a virtual biopsy and can reveal heterogeneity of the entire tumor burden. Methods: We reviewed our prior evaluations of tumor heterogeneity with FES-PET from 3 different studies. 46 breast cancer patients with metastatic disease (de novo or recurrent) who had biopsy proven ER+ primary breast cancer underwent FES-PET and FDG-PET imaging and a biopsy of a metastatic lesion prior to therapy initiation. ER and HER-2 expression was reviewed. Results: Of the 46 patients, 5 (11%) had ER- metastatic biopsies. One (2%) biopsy changed from ER+/HER-2 neg to ER-/HER-2+, and one (2%) biopsy changed from ER+/HER-2+ to ER+/HER-2-. All 5 patients (11%) with changes in ER/HER2 expression underwent a change in therapy due to the unexpected findings by metastatic biopsy. FDG findings helped to guide selection of biopsy sites. FES quantitative measures correlated with biopsy findings. Conclusions: Biopsy resulted in a change in therapy for > 10 % of patients enrolled in trials of FES imaging. Imaging can help identify heterogeneous tumor locations to assist identification of evolving tumor targets in breast cancer. In addition, FES imaging may reveal a change in tumor phenotype that can ultimately affect choice of therapy. Research Support: P01CA42045, R01CA72064