An ovarian Brenner tumor from a 39-year-old woman exhibited striking microcystic change that was so extensive it largely obscured the nature of the tumor, causing initial diagnostic difficulty. The patient presented with symptoms related to a pelvic mass and had a unilateral, 24 cm in maximal dimension, stage I tumor that was solid and cystic. Microscopic examination revealed extensive microcystic change within the epithelial nests in large areas of the tumor such that transitional-type epithelium was only recognized very focally. The tumor had minor areas (<5%) that showed marked cytological atypia and mitotic activity, interpreted as microscopic foci of intraepithelial carcinoma, but was predominantly benign. The presence of characteristic fibromatous stroma and mucinous metaplasia within the epithelial nests provided important clues to the correct diagnosis.