A 44-year-old woman was found to have a sessile lesion replacing most of her endometrium and endocervical mucosa, consisting of an intimate admixture of endometrial glands, endometrial stroma, and smooth muscle. On the basis of a previous curettage, it was concluded that the lesion had been present for at least seven years. The lesion is considered to be related to the pedunculated adenomyomatous polyp. The possibilities that it may represent adenomyosis, neoplasm, or hamartoma are discussed, and the latter is considered to be the most likely possibility. The presence of smooth muscle distinguishes the present case from the recently described and probably closely related entities of papillary adenofibroma, benign müllerian tumor, and miillerian adenosarcoma.