Adenomatoid tumors arising in the uterus are not well-recognized and are sometimes mistaken for other benign or malignant neoplasms. This study is a clinicopathologic analysis of 12 uterine adenomatoid tumors. Characteristically, they were small nodular lesions (mean diameter 2.1 cm) in the subserosa or outer wall of the myometrium. Four distinctive histologic types (adenoid, angiomatoid, solid, and cystic) were identified; each tumor consisted of a combination of one major and at least one minor type. The nodular proliferation of smooth muscle which accompanied some tumors was regarded as a non-neoplastic hyperplasia of myometrium. In two cases, small foci of serosal endosalpingiosis were adjacent to the tumor. Theories of histogenesis are discussed, and classification of the adenomatoid tumor as a type of benign mesothelioma is endorsed.