The degree of urothelial differentiation in putative transitional (urothelial) proliferations in the female genital tract is still controversial. To further investigate the similarities (or dissimilarities) between female genital tract transitional proliferations and bladder urothelium, we evaluated the expression of S100P and GATA3, 2 proteins that we previously found to be strongly expressed in bladder urothelial tumors, in 25 benign ovarian Brenner tumors, 19 Walthard cell nests (17 tubal and 2 ovarian hilus), 1 mature teratoma with a benign urothelial proliferation, 2 proliferating (borderline) ovarian Brenner tumors, 1 malignant Brenner tumor, and 12 ovarian transitional cell carcinomas (TCC). Each lesion was also evaluated for p63 expression by immunohistochemistry. Immunostaining was performed on formalin-fixed, paraffin-embedded tissue sections using the avidin-biotin-peroxidase complex method. Eighty-eight percent of Brenner tumors were positive for S100P, whereas 96% and 100% were positive for GATA3 and p63, respectively. One of 2 proliferating Brenner tumors was positive for S100P, whereas both cases were positive for GATA3 and p63; the malignant Brenner tumor was positive for S100P and p63, but negative for GATA3. Only 17% of TCC were positive for S100p, whereas 33% and 50% of TCC were positive for GATA3 and p63, respectively. Tubal Walthard cell nests were either completely negative or showed only scattered positive staining for S100P; in contrast, 89.5% and 100% of Walthard nests, including the 2 ovarian cases were positive for GATA3 and p63. The teratoma-associated benign urothelial proliferation was also negative for S100P, but positive for GATA3 and p63. Although proliferating and malignant Brenner tumors may exhibit a more intermediate immunoprofile, expression of S100P, GATA3, and p63 by a majority of ovarian Brenner tumors underscores the similarity between these neoplasms and urothelial proliferations of bladder origin. The indeterminate phenotype seen in Walthard nests and ovarian TCC suggests that these proliferations may represent an incomplete or alternate form of differentiation.