Two cases of müllerian adenofibroma of the uterus with unusual features are described. The tumors were encountered in patients 51 and 70 years of age who presented with abdominal pain and postmenopausal bleeding, respectively; both patients were found to have a pelvic mass. Intraoperative findings included an enlarged uterus in each case, and in one of them, uterine rupture and worm-like plugs of tumor within veins of both broad ligaments. Examination of the hysterectomy specimens revealed polypoid endometrial tumors that deeply invaded the myometrium. On histologic examination, both tumors were composed of an intimate admixture of benign endometrial-type glands and a moderately cellular stroma containing fibroblasts with benign nuclear features and very low mitotic activity (less than one mitotic figure per 10 high-power fields). Both tumors infiltrated the myometrium almost to the serosa, and one of them grew within the lumens of large myometrial veins. The patients had uneventful postoperative follow-up periods of greater than 3 years. This report indicates that rare, otherwise typical uterine adenofibromas can exhibit invasive properties and may have a malignant potential.