To assess the long-term effect of Dienogest on clinical complaints and nodule's size in women affected by rectosigmoid deep endometriosis (DE). This is a single centre longitudinal prospective observational study comprising a consecutive series of women affected by rectosigmoid DE, who underwent medical treatment with Dienogest (2mg daily continuous). All women underwent clinical visit and transvaginal sonography (TVS) with bowel preparation prior to start therapy and at 3-6 months' interval for at least 12 months. Clinical complaints such as dysmenorrhea, dyspareunia and dyschezia were assessed using a Visual Analogue Scale (VAS). DE rectosigmoid lesion was measured in the three-orthogonal planes. Lesion's volume was estimated using the prolate ellipsoid formula. The maximum diameter and lesion volume were used for analysis. Patient's complaints and lesion size before starting the treatment and at 12 months were compared. Over a three-year period (January 2017-December 2019), 92 patients were recruited. Patients' mean age was 36.7 years (range: 20 to 50 years). We did not observe a significant correlation between the severity of the symptoms and lesion size prior to start therapy. Clinical complaints improved significantly during treatment. However, lesion size and volume did not change significantly (table 1). Dienogest reliefs significantly clinical symptoms related to rectosigmoid DE. However, this treatment does not exert a significant effect on lesion's size as assessed by TVS after a long-term therapy.