Purpose: The purpose of this study was to investigate whether positron emission tomography (PET) with the glucose analog [18F]fluorodeoxyglucose (FDG) and the estrogen analog 16 alpha [18F]fluoroestradiol-17 beta (FES), performed before and after treatment with tamoxifen, could be used to detect hormone-induced changes in tumor metabolism (metabolic flare) and changes in available levels of estrogen receptor (ER). In addition, we investigated whether these PET findings would predict hormonally responsive breast cancer. Patients and Methods: Forty women with biopsy-proved advanced ER-positive (ER+) breast cancer underwent PET with FDG and FES before and 7 to 10 days after initiation of tamoxifen therapy; 70 lesions were evaluated. Tumor FDG and FES uptake were assessed semiquantitatively by the standardized uptake value (SUV) method. The PET results were correlated with response to hormonal therapy. Results: In the responders, the tumor FDG uptake increased after tamoxifen by 28.4% ± 23.3% (mean ± SD); only five of these patients had evidence of a clinical flare reaction. In nonresponders, there was no significant change in tumor FDG uptake from baseline (mean change, 10.1% ± 16.2%; P = .0002 v responders). Lesions of responders had higher baseline FES uptake (SUV, 4.3 ± 2.4) than those of nonresponders (SUV, 1.8 ± 1.3; P = .0007). All patients had evidence of blockade of the tumor ERs 7 to 10 days after initiation of tamoxifen therapy; however, the degree of ER blockade was greater in the responders (mean percentage decrease, 54.8% ± 14.2%) than in the nonresponders (mean percentage decrease, 19.4% ± 17.3%; P = .0003). Conclusion: The functional status of tumor ERs can be characterized in vivo by PET with FDG and FES. The results of PET are predictive of responsiveness to tamoxifen therapy in patients with advanced ER+breast cancer.