Uterus-like mass replacing ovary: report of a new entity.

A unique, uterus-like ovarian mass with central endometriosis was studied. A 31-year-old woman with a ten-year history of endometriosis of the left ovary had surgically excised a left ovarian mass with a central cavity that was lined by endometriotic tissue, which was surrounded in turn by a very thick wall of smooth-muscle cells that contained a scanty amount of collagen. The general architecture of the mass simulated that of a uterus. The tentative hypothesis is that the present mass was the end result of an intensive and complete transition of the ovarian stromal cells into smooth-muscle fibers.