This is the report of three cases of unsuspected uterine leiomyosarcoma diagnosed by pathologist after hysteroscopic resection. The literature on this issue has been reviewed. Mesenchymal uterine tumors are rare malignancies, occurring in only 17 per one million women annually. The three most common variants of uterine sarcoma are endometrial stromal sarcoma, leiomyosarcoma, and malignant mixed Müllerian tumour. Less than one percent of women believed to have a leiomyoma actually have a sarcoma at hysterectomy. According to the authors' experience and the available literature reviewed, the removal of the whole myomatosus lesion, even if its appearance suggests a typical submucosal myoma, represents the only method to definitively rule out the presence of sarcomatous tissue.