We aimed to evaluate treatment responses and recurrence rate of atypical endometrial hyperplasia (AEH) and endometrial endometrioid adenocarcinoma (EA) with Stage IA Grade 1 to megestrol in Iranian patients who are candidates for medical treatments. Methods: In a retrospective cohort study that was conducted on 50 patients with AEH and 22 patients with EA who were referred to the oncology clinic of Imam Khomeini Hospital, Tehran, Iran, during 2006–2016, we recruited all patients with AEH or EA of Stage IA Grade 1 and their disease was diagnosed during endometrial curettage with or without hysteroscopy. Patients were initially treated with 160 mg of megestrol daily, along with aspirin up to 3 months, and then after 3–4 weeks of discharge of the drugs, patients underwent curettage with hysteroscopy. Findings: The patients with AEH had 31 complete responses and five progressive diseases, and the patients with EA had seven complete responses and seven progressive diseases. After treatment, 25 cases with AEH and 5 cases with EA had an intention to get pregnant, whereas eight patients with AEH and 1 case with endometrial cancer became pregnant. Recurrence occurred in the 2 cases with AEH and 2 cases with endometrial cancer which the time of recurrence in the patients with AEH was longer than in patients with endometrial cancer (P = 0.011). Conclusion: Megestrol is an effective therapeutic agent in endometrial hyperplasia or low-grade endometrial cancer patients who are willing to conserve their childbearing.