An unmarried girl with severe dysmenorrhea had laparotomy for bilateral endometriomas, later treated with danazol and GnRHa Depo injection for more than three years. On laparoscopy she had a large, noncommunicating, functioning rudimentary horn with ipsilateral renal agenesis. Laparoscopic excision of the rudimentary horn gave major relief from the suspected endometrioma, which was not the cause of her severe pain.