METHODS: We conducted a retrospective study examining uptake in matched lesions for FES and FDG-PET scans, assessing these parameters in 213 patients with ER-positive/HER2-positive (n = 33) versus ER-positive/HER2-negative MBC (n = 180). We employed log-rank and t-tests to assess the association of HER2 status with outcome variables and the hypotheses that patients expressing HER2-positive disease lived longer than patient with HER2-negative disease. RESULTS: No difference in FES or FDG avidity was observed between patients with HER2-negative or HER2-positive tumor status. Limited data also suggests that patients with HER2-positive disease had better overall survival (p = 0.024), than those with HER2-negative disease, but not time-to-progression between the same patient cohorts. CONCLUSION: This retrospective analysis suggests that there is a possible role for future trials using FES-PET in helping to select patients with ER+/HER2-positive primary tumors who retain ER expression at all sites of disease and may benefit from endocrine therapy.