To determine whether proactive molecular risk classifier for endometrial cancer (ProMisE) could be used to assess the prognosis of patients with atypical endometrial hyperplasia (AEH) or early-stage endometrial cancer (EC) treated with levonorgestrel-releasing intrauterine system (LNG-IUS).A retrospective cohort study was conducted among 93 AEH or early-stage EC patients who received LNG-IUS to preserve fertility . By immunohistochemistry and gene sequencing, 4 subtypes of ProMisE were identified (p53 wild type [p53 wt], mismatch repair-deficient [MMRd], p53-abnormal, and POLE-mutated). The primary outcome was the time to complete response (CR) after LNG-IUS therapy. Secondary outcomes included the recurrence rate after CR and success rate of conception.Among the 93 patients, 15 (16.1%) were classified as MMRd, 6 (6.5%) as POLE-mutated, 5 (5.4%) as p53-abnormal, and 67 (72.0%) as p53 wt. Comparison of serum cancer antigen 125, family history of tumor, and positive rates of programmed cell death 1 ligand 1 protein and Ki67 protein in 4 groups showed statistically significant differences (p<0.05). Patients with the p53-abnormal subtype had the lowest overall CR rate (40%) and the highest recurrence rate (2/2). Patients with POLE-mutated subtype had the best prognosis, and all 6 patients achieved CR. When patients achieved complete remission, assisted reproductive technology was more likely to help them conceive than natural conception (p<0.05).Patients with early-stage EC or AEH who are more likely to benefit from fertility-sparing treatment can be identified using ProMisE classifier. Patients with POLE-mutated are suitable for fertility-sparing treatment with LNG-IUS.© 2024. Asian Society of Gynecologic Oncology, Korean Society of Gynecologic Oncology, and Japan Society of Gynecologic Oncology.