Ovarian tumors of low malignant potential (LMP) must be distinguished from benign, "proliferating" ovarian tumors and from frank ovarian carcinoma. Serous and mucinous tumors of LMP demonstrate epithelial stratification, the formation of epithelial tufts, cytologic atypia, and mitotic activity, but they do not demonstrate stromal invasion by epithelial cells, which is a feature of frank carcinoma. Mucinous carcinoma may also be recognized by epithelial stratification exceeding three cell layers and the formation of true cribriform glandular patterns. Although controversial, we do not at present recognize a LMP tumor of endometrioid type but prefer to classify those endometrioid neoplasms with a prominent fibrous stroma and glandular complexity similar to adenomatous endometrial hyperplasia as proliferating endometrioid adenofibromatous and cystadenofibromatous tumors. There is only mild cytologic atypia in such tumors. Because of the moderate to marked cytologic atypia that occurs in some clear cell neoplasms with a prominent fibrous stroma, we believe those tumors do merit a designation of LMP tumors. In both the proliferating endometrioid and LMP clear cell adenofibromatous and cystadenofibromatous tumors, carcinoma must be excluded by an absence of stromal invasion, which is frequently recognized by a confluent glandular pattern. The histologic features of proliferating Brenner tumors are similar to those of low grade, papillary, noninvasive, urothelial carcinoma, whereas we propose that Brenner tumors of LMP show high grade cytologic atypia but remain noninvasive in the ovary.