Tubo-ovarian abscess (TOA), a serious complication of pelvic inflammatory disease, unites the fallopian tube and ovary and, is rarely observed in sexually inactive adolescent girls. A pelvic mass, supposedly originating from the ovary, was detected in a 13-year-old sexually inactive girl suffering from abdominal pain and menstrual disorder. Pelvic ultrasonography pointed out a semisolid, hyperechogenic mass of 57x73 mm in the left adnexal area. Laparotomy revealed an unilateral TOA adhering to the bowel and omentum. Abscess drainage and adhesiolysis were performed and postoperative antibiotherapy was administered. TOA should be considered in the differential diagnosis of females with abdominal pain and adnexal mass whether sexual activity is present or not.