Endometriosis is a gynecological condition in which cells from the lining of the uterus (endometrium) appear and flourish outside the uterine cavity, most commonly on the membrane which lines the abdominal cavity, the peritoneum. The uterine cavity is lined with endometrial cells, which are under the influence of female hormones. Endometrial cells in areas outside the uterus are also influenced by hormonal changes and respond in a way that is similar to the cells found inside the uterus. Symptoms of endometriosis are pain and infertility. The pain often is worse with the menstrual cycle and is the most common cause of secondary dysmenorrhea. Endometriosis was first identified by Baron Carl von Rokitansky in 1860.