16alpha-[(18)F]Fluoro-17beta-estradiol (FES) is an estrogen receptor (ER) ligand used for the detection of ER-positive malignant tumors such as breast cancer. We recently reported the feasibility of combined FES-and 2-[(18)F]fluoro-2-deoxy-D-glucose (FDG)-positron emission tomography (PET) scans for the differential diagnosis of endometrial tumors. ER expression measured by FES-PET was preserved in endometrial hyperplasia, whereas ERs were assumed to be reduced in endometrial carcinoma with accelerated glucose metabolism measured by FDG-PET. We report two postmenopausal patients under suspicion of endometrial carcinoma on the basis of cytology and/or magnetic resonance imaging (MRI), who were on tamoxifen treatment since undergoing surgery for breast cancer. Pelvic MRI suggested endometrial carcinomas, whereas FDG-and FES-PET showed no abnormal tracer accumulation. A postoperative histopathologic examination revealed that the lesions were endometrial hyperplasias with no malignant findings. FES-PET enables us to evaluate the ERalpha expression of endometrium noninvasively, whereas the evaluation of ER expression using FES-PET requires careful attention regarding the influence of hormonal therapy because tamoxifen greatly affects FES accumulation of even endometrial hyperplasia, which should be an FES-avid lesion.