Ten benign biphasic cervical tumors that we have designated "adenomyomas of endocervical type" are reported because they might be confused with adenocarcinoma. The patients ranged from 21 to 55 years of age (mean, 40 yr). Two presented with abnormal vaginal bleeding, but, in most patients, the cervical tumors did not cause symptoms. On physical examination or at operation, eight patients were found to have tumors ranging from 1.3 to 8.0 cm in greatest dimension growing into the endocervical canal and, in three cases, prolapsing through the external os. The remaining two patients had mural tumors measuring 11.0 and 23.0 cm in greatest dimension, which projected into the pelvis from the outer aspect of the cervix, without mucosal involvement. The tumors were well circumscribed and grey-white or tawny, and five contained multiple mucin-filled cysts up to 3.0 cm in diameter. One tumor was focally hemorrhagic. On microscopic examination, the tumors were composed of glands and cysts lined by a single layer of endocervical-type mucinous epithelium admixed with smooth muscle. The epithelial component was typically composed of large irregularly shaped glands with papillary epithelial infolding, surrounded by smaller simple glands, frequently resulting in a lobular arrangement. Tubal-type epithelium was present focally in six tumors and endometrial-type glands surrounded by endometrial stroma were present in one case. Both the epithelium and smooth muscle were uniformly bland, without significant mitotic activity. Five patients were treated initially by "polypectomy." Hysterectomy 1 month and 1 year later in two of the cases revealed residual adenomyoma; a "recurrence" 3 years after polypectomy in another patient was treated by hysterectomy. In no case has there been evidence of spread beyond the cervix. The finding of a cervical tumor composed of bland, irregularly shaped, mucinous glands surrounded by smooth muscle caused significant problems in differential diagnosis and a diagnosis of adenoma malignum was either favored or raised as a possibility by the initial pathologist in five of the cases. The gross circumscription of the adenomyomas, their polypoid appearance, the frequent lobular arrangement of glands, the absence of invasive glands with a desmoplastic stromal reaction, and lack of even focal atypia were the most helpful findings in differentiating these tumors from adenoma malignum.