MDM2 amplification is known to occur in a variety of neoplasms and its detection by fluorescence in situ hybridization is helpful in distinguishing well-differentiated and dedifferentated liposarcoma from classic lipoma. We recently evaluated a mesenteric mass initially diagnosed as dedifferentiated liposarcoma, largely due to the neoplasm's myxoid morphology and MDM2 expression by immunohistochemistry, from a 46-yr-old woman with a history of uterine low-grade endometrial stromal sarcoma (LG-ESS) with a JAZF1 rearrangement. Our workup of the mesenteric mass revealed a JAZF1 rearrangement and a revised diagnosis of metastatic LG-ESS with myxoid change was rendered. Retrospective testing of the mesenteric mass was negative for MDM2 amplification, an uncommon, but known diagnostic pitfall in MDM2 expression by immunohistochemistry. As MDM2 amplification is not specific for the diagnosis of liposarcoma, we investigated its occurrence in 43 cases of endometrial stromal tumors: 14 uterine LG-ESS, 11 metastatic or recurrent uterine LG-ESS, 8 undifferentiated uterine sarcomas, 5 endometrial stromal nodules, and 4 high-grade ESS with YHWAE rearrangement. In addition, 40 of the 43 cases had previously undergone fluorescence in situ hybridization analysis of JAZF1, PHF1, and YHWAE. Two of the 43 cases (5%) had MDM2 amplification: one was a uterine LG-ESS (JAZF1 rearrangement) and the other was a undifferentiated uterine sarcoma (polysomy intact JAZF1, PHF1, and YHWAE), both metastatic to the lung. Both cases positive for MDM2 amplification showed MDM2 expression by immunohistochemistry. At last follow-up, both patients had died of disease (19 and 60 mo). Our study is the first to demonstrate MDM2 amplification in endometrial stromal tumor. Awareness of MDM2 amplification in endometrial stromal tumor is critical; particularly in locations more common to liposarcoma, to avoid diagnostic errors.