The histogenesis of so-called adenomatoid tumors has been contested. Doubts as to a mesothelial derivation often have been based on the belief that adenomatoid tumors originate exclusively in or nearby genital tract organs. This report documents a histologically typical adenomatoid tumor which developed in the mesentery of the small intestine. Extensive clinical investigations and an uneventful 3-year postoperative follow-up interval confirmed that the tumor was not a metastatic adenocarcinoma or malignant mesothelioma. The extragenital site of origin of this neoplasm provides additional corroborative evidence for the mesothelial origin of adenomatoid tumors. Regardless of their anatomic location, these distinctive neoplasms should be classified as benign adenomatoid mesotheliomas.