Selective progesterone receptor modulators (SPRMs) have beneficial effects in reducing the size of uterine fibroids and the amount of bleeding, but their endometrial effects have not been seen with other agents. This report describes the morphology of the endometrium after 3 mo of treatment with the SPRM, ulipristal acetate (UPA). In 2 Phase III randomized double-blind controlled clinical trials, 546 patients with uterine myomas were treated with 5 or 10 mg of UPA daily for 13 wk or placebo or gonadotropin-releasing hormone agonist. Endometrial biopsies were taken at screening, end of treatment (13 wk), and after treatment-free follow-up (38 wk). Biopsies were assessed independently by 3 pathologists according to a preset morphologic scheme. After 13 wk, the UPA-treated endometrium showed altered architectural glandular features including extensive cystic dilatation. The glandular epithelium appeared inactive or contained abortive subnuclear vacuolization, occasional mitoses, and apoptosis. Abnormal stromal vessels were commonly seen. There was a high level of agreement between pathologists on the presence or the absence of nonphysiological changes. One case of hyperplasia without atypia and 4 polyps were seen at 13 wk of UPA treatment. Six months after treatment, the endometrium returned to normal histology in the majority of the patients, with 1 polyp and no cases of hyperplasia in the UPA-treated groups, and 2 hyperplasias (1 with and 1 without atypia) in the placebo or the gonadotropin-releasing hormone-agonist groups. Mild reversible thickening of the endometrium occurs in a minority of cases. It is important that pathologists are aware of the spectrum of changes induced by SPRMs to avoid misdiagnoses of endometrial hyperplasia or polyps.