About 5% of Brenner tumors are borderline or malignant. The mean age at presentation is around 60 years. Most patients present with a pelvic mass or pain. The borderline Brenner tumors are unilateral and organ-confined. Grossly, they are large masses with a prominent cystic component containing papillomatous projections into the lumen. Some tumors are entirely solid. Borderline Brenner tumors are composed of transitional epithelium with proliferative changes. They have papillary and polypoid areas that resemble low-grade papillary urothelial carcinoma architecturally. Some cases show large, tortuous nests and ribbons of transitional epithelium arranged back to back. Mild to moderate cytologic atypia, focal necrosis and increased mitotic activity may be present; however, by definition, there is no stromal invasion. Cases with severe cytologic atypia in the absence of invasion are diagnosed as borderline Brenner tumor with intraepithelial carcinoma. Such cases should be thoroughly sampled to rule out the presence of a small invasive component. Focal mucinous metaplasia is commonly present. Solid foci composed of benign Brenner tumor are also seen.