Background: As atypical polypoid adenomyoma (APA) has been reported to be a hormone-related tumor, we aimed to analyze the efficacy and safety of maintenance hormonal therapy after fertility-preserving treatment of these patients with medroxyprogesterone acetate (MPA). Methods: Data were retrospectively analyzed from patients with APA who were treated with a fertility-preserving regimen including MPA between October 2001 and December 2011. Eighteen patients were treated with MPA and 14 (77.8%) achieved either a complete or a partial response after the planned treatment. Five patients took progestin for maintenance therapy. Results: Eighteen patients were treated for a mean observation period of 96.7 months. While taking the maintenance therapy, no patient had APA relapse. One patient developed welldifferentiated endometrioid adenocarcinoma 18 months after she stopped taking maintenance progestin. Eleven patients without maintenance therapy underwent hysterectomy, andnine of them developed well-differentiated endometrial cancer. Through univariate analysis, there was a significant difference in time to hysterectomy between patients with and without maintenance therapy (P = 0.015). Through multivariate analysis, body mass index (BMI), menstrual status before protocol therapy, maintenance treatment, and pregnancy were found to be significantly associated with a lower risk of hysterectomy. No patient had a recurrence of APA after hysterectomy during the observation period (median, 54 months; range, 2-148 months). Conclusion: No patient showed progression while receiving hormonal therapy, including initial protocol therapy. Maintenance hormonal therapy after treatment with MPA was highly effective and safe, particularly in patients with BMI ≧24 kg/m 2 and irregular menstruation cycle.