A 19-year-old woman at 12th week of pregnancy was referred to our hospital with severe acute abdominal pain, nausea and vomiting. Patient's abdomen was untreatable. Routine examinations, except neutrophilic leukocytosis, were normal. Ultrasound imaging showed the presence of intrauterine pregnancy corresponding to amenorrhea and a right complex ovarian mass, with well defined margins without papillary projection, 14x12 cm in diameter. No free fluid was evident. Due to untreatable abdomen and suspecting torsion of the ovarian tumor the patient underwent laparotomy under general anesthesia. The pregnant uterus was normal. The right ovary had increased volume for a cystic-solid mass of the size of 14 cm with smooth surface and twisted on the vascular peduncle. A right adnexectomy was performed. Intraoperative frozen section was negative for malignancy and postoperative histological examination revealed a struma ovarii without atypia of the follicular cells. Postoperative course was normal. The patient was discharged after three days and she delivered at term a normal baby. Mostly struma ovarii represents an incidental finding during cesarean section, but in rare case it may be cause of complications such as torsion, rupture, hypertiroidism and rarely may be a malignant tumor. The AA describe literature data on struma ovarii diagnosed in pregnancy.