Abstract: Estrogen plays an important role in breast cancer by binding with estrogen receptor(ER).There are two ER subtypes(α and β).16α-¹⁸F-fluoroestradiol (¹⁸F-FES) imaging can detect biologically active ER in breast cancer. This nuclear imaging technique can show the effectiveness of antiestrogen treatment and has deep implications on prognosis of breast cancer. Quantitative ¹⁸F-FES uptake and antiestrogen treatment response are significantly associated; baseline ¹⁸F-FES imaging demonstrated that tumors in the patients who responded to tamoxifen had a higher mean standard uptake value for ¹⁸F-FES than the nonresponders. After tamoxifen treatment,the mean tumor ¹⁸F-fluoroestradio uptake decreased consistently with binding of tamoxifen and its bioactive metabolites to ERs. While in the metabolic flare responders, the tumor ¹⁸F-fluorodeoxyglucose uptake increased 7 to 10 days after tamoxifen treatment