Background Selective progesterone receptor modulators have shown efficacy in the treatment of endometriosis, however they are not currently licensed in Canada for this purpose. Their histomorphological effects have been well documented in the endometrium, but not in endometriosis. Case report The patient presented with uterine fibroids and heavy menstrual bleeding, and was treated with ulipristal acetate over three 90-day courses. She eventually elected to undergo definitive surgical management. On pathological examination, she was found to have a focus of endometriosis in the para-tubal soft tissue showing a morphology similar to progesterone receptor modulator associated endometrial changes (PAEC). Conclusions To our knowledge this is the first reported case of PAEC involving a focus of endometriosis. This case demonstrates histomorphological evidence of the effects of ulipristal acetate on endometriosis, and thus provides evidence of a pharmacological effect.