Currently, the indications for progestin therapy are limited to endometrioid adenocarcinoma that are International Federation of Gynecology and Obstetrics (FIGO) grade 1, FIGO stage IA, and confined to the endometrium. However, there have been attempts to broaden the indications of progestin therapy to patients with higher FIGO grades and/or with superficial myometrial invasion. We experienced a case with myoinvasive endometrioid adenocarcinoma treated with oral progestin, whose follow-up endometrial curettage specimen showed an apparent complete histologic regression; however, the final hysterectomy specimen disclosed myoinvasive endometrioid adenocarcinoma within the superficial myometrium, with absence of residual tumor in the endometrium. We describe this case to demonstrate that complete histologic regression of the endometrial lesion in a follow-up curettage specimen after progestin treatment does not guarantee histologic regression of the carcinoma within the myometrium. Our case indicates that current indications for progestin treatment should not be broadened to patients with superficial myometrial invasion.