AbstractLeiomyomas are benign smooth muscle tumors, and retroperitoneal leiomyomas without coexisting uterine leiomyomas are extremely rare. Mitotically active leiomyomas, which are leiomyomas with increased mitotic activity, are rarely observed in postmenopausal women, except under the influence of exogenous hormones. This report presents a rare case of a retroperitoneal mitotically active leiomyoma in a postmenopausal woman. The patient presented with an abdominal mass and underwent surgical resection of the retroperitoneal tumor. Pathological examination revealed a mitotically active retroperitoneal leiomyoma with 31 mitotic figures per 10 high-power fields. The patient did not experience recurrence during the 2-year follow-up period. This case highlights the need to consider retroperitoneal mitotically active leiomyomas in postmenopausal women and suggests that myomectomy can prevent their recurrence.