Inflammatory myofibroblastic tumors of the uterus are rare, and although most have a favorable prognosis, a small subset exhibit extrauterine disease, recur, or cause death. In this study, we evaluated the morphology and immunoprofile of 13 uterine inflammatory myofibroblastic tumors, including four with aggressive behavior. ALK rearrangements were detected by fluorescence in situ hybridization and fusion partners by anchored multiplex assay. Patients ranged from 8 to 63 (mean 39) years and tumors from 2.5 to 20 (mean 7.4) cm. Myxoid, compact, and hyalinized patterns were noted in 13, 12, and 2 tumors, ranging from 1 to 100%, 5 to 99%, and 0 to 5%, respectively. Nuclear atypia was mild in six (46%), moderate in five (38%), and severe in two (15%), with ganglion-like cells in two tumors. Mitoses ranged from 0 to 24 (mean 5) per 10 high-power fields. Inflammation was mild in five (38%), moderate in three (23%), and marked in five (38%), consisting of a lymphoplasmacytic infiltrate that was lymphocyte-predominant in six (46%). Lymphovascular invasion was noted in two (15%) and necrosis in eight (62%). All but one tumor were ALK-positive by immunohistochemistry, with granular cytoplasmic staining in nine (82%). ALK rearrangements (tested in 10) were detected in eight and was absent in one. The remaining tumor showed an isolated green 5' ALK signal. Fusion partners were identified in 10 (77%) and included THBS1 (n=3), IGFBP5 (n=2), DES (n=2), SEC31 (n=1), TPM3 (n=1), and TIMP3 (n=1). Size ≥8 cm was predictive of aggressive behavior (P<0.01), with increased mitoses (≥7 per 10 high-power fields), lymphovascular invasion, and compact-predominance approaching statistical significance. These data show that inflammatory myofibroblastic tumors of the uterus are morphologically heterogenous with frequent ALK expression and a variety of ALK fusion partners. Recognition of this rare mesenchymal neoplasm is crucial as those with aggressive behavior can potentially be treated with tyrosine kinase inhibitors.