We report 30 uterine tumors composed of an admixture of endometrioid glands, endometrioid stroma, and smooth muscle that lacked the characteristic features of atypical polypoid adenomyoma. The patients ranged from 26 to 64 (median 47) years of age. The usual presenting symptom was abnormal vaginal bleeding, which was “massive” in two patients. Six patients were treated by polypectomy only, with hysterectomy performed in the remainder. Twenty-seven adenomyomas were in the corpus (22 submucosal, two mural, and three subserosal) and three in the cervix. The subserosal and submucosal examples were polypoid. The tumors were 0.3 to 17 cm in greatest dimension, and firm with cystic areas often present on sectioning. Focal hemorrhage was described in five cases. On microscopic examination, the tumors were composed of glands and cysts lined by endometrial-type epithelium separated by endometrial stroma and smooth muscle, with smooth muscle predominating. Minor foci of tubal-type epithelium (14 cases), mucinous endocervical-type epithelium (2 cases), and squamous epithelium (1 case) were present. The smooth muscle component was cellular in three cases and contained occasional bizarre nuclei in three cases. The epithelial cells were uniformly bland. No mitotic activity was observed in the epithelial or mesenchymal elements in 20 cases. In the remainder, up to 5 mitotic figures/10 high-power fields were observed in the epithelium (3 cases), the stroma and smooth muscle (3 cases), or both compartments (4 cases). Follow-up in 14 cases revealed no recurrence or extrauterine spread in any case. A diagnosis of adenocarcinoma or adenosarcoma was entertained by the submitting pathologist in five of 14 consultation cases. Adenomyomas are unusual benign uterine tumors that can be misdiagnosed, in part, because the lesion has not often received attention in the literature. The most realistic considerations in the differential diagnosis are atypical polypoid adenomyoma and adenosarcoma. The former, by definition, has epithelial atypia and the latter a malignant (usually low grade) stromal component with typically absent or inconspicuous smooth muscle. Distinction of adenomyoma from adenosarcoma may have significant therapeutic implications, particularly in young women.