16 alpha-F-18-fluoro-17 beta-estradiol (F-18-FES) is an estrogen receptor (ER)-specific PET tracer with various potential interesting applications. The precise contribution of this technique in current clinical practice, however, has yet to be determined. Therefore, the aim of this study was to evaluate the value of F-18-FES PET in breast cancer patients presenting with a clinical dilemma. Methods: F-18-FES PET examination could be requested by referring physicians for patients with a history of ER-positive breast cancer and the presence of a clinical dilemma despite complete standard work-up. All requests for F-18-FES PET required a positive arbitration by a dedicated medical oncologist and nuclear medicine physician. The referring physician was asked to fill in validated questionnaires before, shortly after, and at more than 3 mo after F-18-FES PET to determine indication, diagnostic value, and therapeutic consequences of F-18-FES PET. To further validate F-18-FES PET findings, F-18-FES PET lesions were quantified and compared with centrally reviewed conventional imaging. Results: Thirty-three patients underwent F-18-FES PET between December 2008 and October 2010. F-18-FES PET was requested to evaluate equivocal lesions on conventional work-up (n = 21), ER status in metastatic patients (n = 10), and the origin of metastases (n = 2). F-18-FES-positive lesions were observed in 22 patients. F-18-FES PET was especially sensitive for bone metastases, detecting 341 bone lesions, compared with 246 by conventional imaging. The sensitivity for liver metastases was poor, and quantification of F-18-FES uptake in liver lesions was hampered by high physiologic background. F-18-FES uptake was highly variable between all metastases (range of standardized uptake value, 1.20-18.81), and 45% of the patients with a positive F-18-FES PET finding had both F-18-FES-positive and F-18-FES-negative metastases. F-18-FES PET improved diagnostic understanding in 88% of the patients and led to therapy change in 48% of the patients. Conclusion: With the exception of liver metastases, whole-body imaging of ER expression with F-18-FES PET can be a valuable additional diagnostic tool when standard work-up is inconclusive. F-18-FES PET supported therapy decisions by improving diagnostic understanding and providing information on ER status of tumor lesions.