Leiomyomas are the most common benign uterine tumors. Although 20-40% of all women in reproductive age may have uterine leiomyomas, they are not very common in pregnancy. Only 0.3-2.6% af all pregnant women are diagnosed with leiomyomas. The leiomyomas are symptomatic in 20-50% of all cases. Clinical symptoms are usually excessive or irregular menstrual bleeding, problems resulting from adjacent organs pressure, sterility, miscarriage or problems during the labour. Leiomyomas are known to have estrogen receptors and can demonstrate an extensive growth in high estrogens concentration environment. During the pregnancy they can grow, stay the same size or as well decrease. Rapid leiomyoma’s growth, caused by its transformation into sarcoma, takes place in about 0.1- 0.8% of all cases. In this article we present a case of patient with leiomyoma, which rapid growth, which imitated ovarian tumor. A 40-year old patient was admitted to the 1st Department of Obstretrics and Gynecology, Medical University of Warsaw, in May 2012 because of a large abdominal tumor. She had four vaginal deliveries and one cesarean section. The patients delivered three months before admission. Since the labour she had suffered from dysuria and noticed a quick waits enlargement. On admission the patient was in good general condition, without any stomachache. A giant tumor in her lower and middle abdomen was found. The tumor reached three fingers above the navel. In the ultrasound scan a large solid-cystic the tumor with moderate vascularization was described. It looked like the ovarian neoplasm. The CA-125 plasma concentration was 389,5 IU/ml. After giving a written informed consent the patient had an operation. During the operation a solid-cystic peducled uterine tumor was diagnosed. The diameter of the tumor was about 25 cm. The uterine and uterine appendages had no pathological macroscopic changes. Intraoperative histopathological examination was carried out and revealed mesenchymal tumor without evident polymorphism or mitosis. Because of lack of any signs of malignancy during the operation only the tumor with its peduncle was removed. The patient was discharged in good general condition after three day of postoperative hospitalization. The final histopathological examination revealed leiomyoma with cystic degeneration changes. A small focus of necrosis and extravasation was found. 4 weeks after the operation there were no abnormalities in the gynecological examination and the CA-125 concentration was 27 IU/ml. The presented case illustrates diagnostic difficulties, which may occur when atypical rapid tumor enlargement and its ultrasound image and laboratory tests results imitate ovarian tumor.