Eighteen patients with mesonephroid ovarian carcinoma were investigated. Typical findings for the mesonephroid tumors were abundant extracellular an some intracellular neutral mucin mixed with sulfate and carboxyl groups, and large amounts of glycogen, especially in clear cells. The latter observation is considered to be an important aid in distinguishing them from other epithelial ovarian carcinomas. Some cells contained intracellular cysts that were covered with stubby microvilli similar to those found on the lumenal surfaces of the larger cysts in the tumors. Simultaneous estrogenic endometrial activity in the post-menopausal patients was frequently observed. The significance of this observation is discussed.