Research question: What is the evolution of adenomyosis on magnetic resonance imaging (MRI) after a 3-month treatment course of daily 5 mg doses of ulipristal acetate (UPA) for symptomatic fibroids? Design: A monocentric prospective pilot study on patients who underwent a 3-month treatment course of UPA for symptomatic fibroids between January 2014 and December 2017. Patients underwent pelvic MRI shortly before (pre-MRI) and after treatment (post-MRI). The diagnosis of adenomyosis on MRI was defined by the observation of intramyometrial cysts and/or haemorrhagic foci within these cystic cavities and/or a thickening of the junctional zone >12 mm. The progression of adenomyosis was defined by the presence of at least one of the aforementioned criteria of adenomyosis on the pre-MRI and by at least one of the following on the post-MRI: (i) increased thickness of the junctional zone ≥20% and/or (ii) increased number of intramyometrial cysts. The appearance of adenomyosis was defined by the absence of the aforementioned criteria of adenomyosis on the pre-MRI and the presence of at least one of these criteria on the post-MRI. Results: Seventy-two patients were included. The MRI features of adenomyosis progressed for 12 of 15 patients (80.0%) for whom adenomyosis was identified on the pre-MRI. An appearance of adenomyosis was identified after treatment for 15 of 57 patients (26.3%) for whom adenomyosis was not identified on the pre-MRI. Conclusions: A 3-month treatment course of daily 5 mg doses of UPA could provoke a short-term progression or an emergence of typical adenomyosis intramyometrial cysts on MRI examinations.