Ovarian abscess is a rare gynecologic entity; only 120 cases have been reported in the world literature. In contrast to the more commonly encountered tubo-ovarian abscess, tubal involvement is absent in the ovarian abscess. This report describes a case of an ovarian abscess arising de novo within an endometrioma. The woman presented with a 20-cm pelvic-abdominal mass with paracaval adenopathy and bilateral ureteral obstruction. Treatment with broad-spectrum antibiotics was unsuccessful. The surgical specimen revealed endometrial glands, stroma, hemosiderin-laden macrophages, and evidence of abscess formation.