Uterine tumors resembling ovarian sex cord tumors (UTROSCT) are tumors of unclear histogenesis. The authors analyzed the ultrastructural features of 13 UTROSCT and correlated the findings with their immunohistochemical profile. Features included cells with frequent organoid, nested or cord-like arrangement (8), lumen formation (2; one of which showed surface microvilli), nuclei with irregular indentations (8), intermediate filaments (13), prominent paranuclear aggregates (5), cell junctions (9), desmosome-like junctions (2), tonofilaments (2), basal lamina (1), and cytoplasmic lipid droplets (7; prominent in 3). No dense bodies, subplasmalemmal densities or pinocytotic vesicles were seen. Ultrastructural epithelial differentiation was present in 2 tumors (positive for keratin or epithelial membrane antigen). Prominent lipid droplets correlated with sex cord markers positivity in 2 tumors. Ultrastructural features of smooth muscle differentiation were lacking and abundant paranuclear filaments did not correlate with myoid markers. UTROSCT are polyphenotypic neoplasms ultrastructurally with focal epithelial and variable sex cord-like differentiation. These findings suggest that UTROSCT may result from divergent differentiation in endometrial stromal tumors or represent a distinct group of uterine tumors with sex cord-like differentiation that are closer in histogenesis to ovarian sex cord stromal tumors.