The aim of our study was to evaluate the possibility of ultrasonography in differentiating malignant from nonmalignant cysts in postmenopausal women. The study included 100 postmenopausal women with diagnosis of simple ovarian cysts who were treated surgically at our clinic during the study period. In the group of operated patients the histopathological diagnosis confirmed benign cysts in most cases. In 1% of patients histopathological diagnosis was borderline or malignant tumor of the ovary. The risk of malignancy ranged from 0% in the cyst with a diameter below 50 mm, to 3.57% in cysts over 80 mm. Cyst diameter smaller than 50 mm and the volume of cyst less than 50 cm3 in correlation with the values of serum CA125 and clinical findings represent good algorithm for differentiating malignant from nonmalignant simple cyst.