Ultrasound is now frequently used for evaluation of pathological findings discovered on gynecological examination and for puncture of ovarian cysts. Although the new, high-frequency vaginal transducers have a very high resolution, only macroscopically visible structures of the tumors can be imaged. For this reason, it seemed important to classify ovarian tumors according to their macroscopic appearance and then relate this to whether the tumor was benign, borderline, or malignant. Such a classification has not been performed before. Medical records from women operated upon due to pelvic tumors over a period of 11 years were scrutinized. There were 1017 women included in the study. Among those tumors characterized as unilocular cysts 0.3% (1/296) was malignant; this tumor had macroscopically visible papillary vegetations on the inside of the cyst wall. This cyst was found in a woman 60 years old. Sixty percent (178/296) of the women who had a unilocular cyst were over the age of 40. Two percent (4/203) of the unilocular solid tumors were classified as malignant. The malignancy rates for multilocular cysts was 8% (20/229), multilocular solid tumors 36% (147/209), and solid tumors 39% (31/80). Papillary vegetation on the cyst wall was the structure that was most frequent in malignant tumors. Neither the thickness of the cyst wall nor the thickness of septa inside the tumor seemed to correlate with malignancy. Among the simple ovarian cysts, 65 had a diameter over 10 cm but none of them was malignant. The one that was malignant had a diameter of approximately 5 cm. In conclusion, unilocular ovarian cyst seems to carry a very slight chance of malignancy even in women over the age of 40. Papillary vegetation on the cyst wall, a structure that can be seen by ultrasound, seems to be a serious sign.