Objective: Medroxyprogesterone acetate (MPA) is an effective fertility-preserving treatment for early endometrial cancer and atypical endometrial hyperplasia (AEH), and rarely leads to the development of extrauterine lesions (ELs). We aimed to clarify the characteristics of patients who developed ELs post-MPA therapy. Methods: We analyzed the clinicopathological factors and prognoses of 319 patients with endometrioid carcinoma grade 1 (EMG1) and AEH treated with MPA at our institution. All patients underwent imaging before MPA treatment to rule out ELs. Results: Seven patients (2.2%) with EMG1 showed EL after MPA treatment. Two patients developed EL during the initial treatment, 2 during repeated treatment, and 3 during followup. Two patients had peritoneal dissemination, 3 had regional lymph node metastasis, 1 had distant metastasis at the Virchow lymph node, and 1 had ovarian metastasis. ELs were diagnosed using imaging tests in 6 patients and elevated tumor markers in 3 (overlapping) patients. One patient was diagnosed with ELs pathologically after hysterectomy. Upon EL diagnosis, patients underwent standard treatment, including hysterectomy and chemotherapy, that was followed by a diagnosis of EMG1 for 5, EMG2 for 1, and EMG3 for 1 patient. One patient died 15 months after start of therapy and another died 119 months posttreatment initiation, while the others have been survived. Conclusion Only 2.2% of all patients developed ELs post-MPA treatment, but some cases were fatal. It is essential to conduct imaging tests and screen for tumor markers during and after MPA treatment regularly and also when cancer progression is suspected.