Objective: Up to 60% of patients with a precancerous endometrial lesion will ultimately be diagnosed with endometrial carcinoma. In the context of endometrial carcinoma, adequate surgical staging—including lymph-node assessment—should be performed and dictate the necessity of postoperative adjuvant treatments. Our main objective was to evaluate the diagnostic performance of preoperative Positron Emission Tomography-Computed Tomography (PET/CT) in identifying concomitant endometrial cancer in patients with confirmed precancerous endometrial lesions. Methods: All women diagnosed with a precancerous lesion between 2010 and 2018 in our center were included in this retrospective cohort study. Patients were then divided into groups according to whether or not a PET/CT was performed preoperatively and the presence of endometrial carcinoma at the final pathology. Results: A total of 128 patients met the inclusion criteria, of which 66 underwent PET/CT. The sensitivity of PET/CT in identifying carcinoma was 78.3%, with a specificity of 79.1%. PET/CT failed to identify carcinoma in 5 out of 66 patients (7.6%). In the PET/CT group, 18 of 23 patients (78.3%) had adequate surgical staging, compared to only 4 of 31 patients (12.9%) in the standard group (p 0.00001). Conclusion: Preoperative PET/CT reliably predicted the presence of endometrial carcinoma in women with precancerous endometrial lesions. Future trials should explore the value of adding PET/CT in the preoperative investigation of these patients to identify women who may be offered sentinel-lymph node mapping.