FDG PET has been widely used to delineate the metabolic status of neoplasms for diagnosis, staging, and restaging. However, incidental diagnosis of endometrial carcinoma for multiple primary neoplasms by FDG PET/CT is rare. A 57-year-old woman with left breast cancer underwent whole-body FDG PET/CT examination as a result of elevation of serum titers of tumor markers but free of other clinical symptoms. The image revealed a high-grade uptake of FDG-avid lesion within the endometrium of the uterus, but no abnormal FDG accumulation in the rest of the body. The final pathology disclosed endometrial adenocarcinoma.