Brenner tumors comprise 1 to 2% of ovarian neoplasms; the vast majority are benign and measure less than 2 cm. Microscopically, they are composed of sharply demarcated, solid to partly cystic epithelial cell nests surrounded by a stroma composed of tightly packed, spindle-shaped cells. The epithelial cells are polygonal with pale, eosinophilic cytoplasm and oval nuclei that display distinct nucleoli and longitudinal grooving, the so-called coffee-bean appearance. Walthard cell nests are most commonly found on the peritoneal surface of the Fallopian tubes and also in the mesovarium and occasionally in the ovarian hilus. Because of the histological similarity of the epithelium of Brenner tumors and Walthard cell nests to the urothelium of the lower urinary tract, it has been suggested that Brenner tumors and Walthard cell nests represent urothelial (transitional cell) differentiation. This has further been supported by electron microscopic analysis showing similarities between them, including deeply indented nuclei, sparse granular endoplasmic reticulum, abundant free ribosomes, presence of glycogen particles, secondary lysosomes, moderate numbers of desmosomes, markedly tortuous, interdigitating cell membranes, and large complex intercellular spaces. [1][2][3][4] Both Brenner tumors and normal urothelium immunohistochemically stain for carcinoembryonic antigen 5-7 and involucrin.8 However, none of these features are restricted only to urothelium.The defining characteristic feature of terminal urothelial differentiation is the presence of the asymmetric unit membrane on the luminal surface and in intracytoplasmic fusiform vesicles.9 These structures have not been observed in Brenner tumors or in Walthard nests by ultrastructural examination.