Mesenteric fibromatosis (MF) is a rare benign mesenchymal lesion that can occur throughout the gastrointestinal tract, especially small bowel. Its biological behavior is intermediate between benign fibrous tissue proliferation and malignant fibrosarcoma. In previously reported cases of MF, we could find colonic obstruction or ureter obstruction, but simultaneous involvement of colon and ureter was not able to be seen. We described a patient that presented with colonic obstruction and hydroureteronephrosis due to MF at sigmoid colon which mimicked submucosal tumor such as gastrointestinal tumor. This case resulted in a positive positron emission tomography scan suggesting malignant neoplasm, but β-catenin positivity on immunohistochemical staining separated MF from gastrointestinal stromal tumor and sclerosing mesenteritis. The clinical course of the patient was improved after surgical resection. (Gut and Liver 2009;3:215-217)