Uterus-like ovarian mass presenting near menarche.

An eleven year-old girl presented with right lower quadrant abdominal pain near menarche that coincided with the onset of her most recent menstrual period. A smooth-lined cystic structure with a thick wall was discovered in the right adnexa at surgery that was adherent to a multicystic ovary and a dilated fallopian tube. Histologic examination determined the lesion to be a uterus-like ovarian mass. The patient had a history of a resected right pelvic kidney at four months of age. This case is believed to represent a congenital malformation of the urogenital system rather than smooth muscle metaplasia of ovarian stromal cells.