Endometriosis is defined as the presence of endometrial glands and stroma outside the uterus. The most common site of involvement is ovary. The patients are usually of child-bearing age and present with infertility and cyclical pain associated with menstruation. Repeated bouts of hemorrhage may convert ovary into a cyst filled with brown material (chocolate cyst). Image courtesy of: Dr. Bulent Celasun, Ankara, Turkey. Used with permission.