It is likely that inflammatory myofibroblastic tumor represents a continuum of tumorous lesions characterized by myofibroblastic and fibroblastic proliferation with a prominent inflammatory infiltrate. It has been described in almost every anatomic site, and the majority of cases pursue a benign clinical course; however, anatomic location may dictate biologic behavior somewhat. In particular, retroperitoneal and mesenteric tumors are particularly problematic; rare examples of multiple recurrences with histologic sarcomatous progression and of distant metastasis have been observed from tumors at these locations, and the designation of inflammatory fibrosarcoma perhaps should be reserved for these uncommon cases. Although the precise cause of these lesions remains elusive, with several lines of evidence suggesting either an infectious, inflammatory, or autoimmune origin, cases with clonal cytogenetic abnormalities and aneuploidy have been reported, suggesting that some lesions represent true neoplasms.