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Unlabelled: The purpose of this study was to assess the results of PET with 16 alpha-[18F]fluoro-17 beta-estradiol (FES) and [18F]fluoro-2-deoxy-D-glucose (FDG) to validate the concordance between tumor estrogen-receptor (ER) status as determined by FES-PET and in vitro assays and to assess the relationship between tumor metabolic activity determined by FDG-PET and tumor ER status, both of which may provide information about tumor aggressiveness and prognosis., Methods: We studied 32 patients with primary breast masses and 21 patients with clinical or radiological evidence of recurrent/metastatic breast carcinoma. A diagnosis of breast carcinoma was subsequently proven in 43 patients (24 primary, 15 metastatic and 4 recurrent tumors). In vitro assessment of ER status was available for 40 malignant lesions (23 primary and 17 metastatic/recurrent). The patients underwent PET with both FES and FDG, and the uptake of each tracer within each lesion was evaluated qualitatively as well as semiquantitatively using the standardized-uptake-value (SUV) method., Results: We found good overall agreement (88%) between in vitro ER assays and FES-PET. This degree of agreement is similar to that observed between replicate in vitro assays (with discordances due to interlaboratory, interassay and specimen variability). We were, however, unable to demonstrate any significant relationship between tumor FDG uptake and ER status or between tumor FDG and tumor FES uptake in these patients., Conclusion: These results indicate that in vitro ER assays and/or FES-PET provide unique direct information about breast cancer ER status that cannot be obtained indirectly by FDG-PET.