Uterine tumors resembling ovarian sex cord tumors (UTROSCTs), first characterized by Drs Clement and Scully in 1976, are rare neoplasms showing clinical, morphologic, and immunohistochemical overlap with a number of other uterine tumors, most being mesenchymal. Criteria for aggressive behavior are not clearly established. We report 75 tumors from patients ranging from 21 to 84 (mean=52.4) years. Seventy-one patients were treated by hysterectomy and 4 by conservative total excision. Thirty-eight tumors were intramyometrial, 34 submucosal, and 3 cervical; they ranged from 0.6 to 20 (mean=4.9) cm and were typically tan-yellow. Sixty-eight neoplasms were well-circumscribed and 7 had infiltrative borders (4 only minimally). In 56 tumors, a smooth muscle component was intimately admixed with the neoplastic cells ("pseudoinfiltration"; extensive in 29). Architectural patterns included cords (n=53), diffuse (n=51), hollow tubules (n=48), nests (n=38), trabeculae (n=37), retiform (n=23), solid tubules (n=21), pseudoangiomatoid (n=11), pseudopapillary (n=4), and whorled (n=2); typically, more than 1 pattern was seen. Tumor cells were epithelioid (n=62), epithelioid and spindled (n=12), or spindled (n=1) and/or rhabdoid (n=20; extensive in 2). Cytologic atypia was absent to mild in 57, moderate in 16, and moderate to severe in 2 tumors. Fifty-seven UTROSCTs had ≤2mitoses/10 high power fields (HPF), 12 had 3 to 5/10 HPF, and 6 >5/10 HPF. Necrosis was present in 3 and lymphovascular invasion in 1. Tumor cells showed a polyphenotypic immunohistochemical profile (with positivity for sex cord, smooth muscle, and epithelial markers), most commonly inhibin (17/33+) and calretinin (22/31+) positive. Five of 58 patients with follow-up (22 to 192; mean=73.2 mo) had recurrences/metastases from 30 to 144 months, and 2 died of disease. Malignant tumors showed >3 of the following 5 features compared with benign tumors: size >5 cm, at least moderate cytologic atypia, ≥3 mitoses/10 HPF, infiltrative borders, and necrosis. One of the 5 malignant tumors showed an extensive rhabdoid morphology. UTROSCTs are uncommon, show a wide morphologic spectrum, often pose problems in differential diagnosis, and typically have a benign outcome. Rare tumors are associated with late recurrences and a combination of more than 3 of the 5 features listed above predicted aggressive behavior in this series.