We intended to evaluate ultrasonographic criteria for differentiation of benign and malignant ovarian tumours, and to estimate the risk of malignancy in unilocular ovarian cysts. The files of 186 women aged 40 or above, who had undergone surgery in the department of gynaecology and obstetrics at Hillerød hospital between 01.01.1988-31.12.1990 and where pathological ovarian histology was found were reviewed. Preoperative ultrasonographic examination concerning size and morphology was compared to histological diagnosis. Tumours were classified according to the ultrasonographic internal structure into a) unilocular cysts, b) unilocular cysts with solid areas, c) multilocular cysts, d) multilocular cysts with solid areas and e) solid tumours. Criteria for suspicion of malignancy were unilocular cysts > 10 cm and complex and solid tumours, whereas smaller unilocular cysts were thought to be benign. Twenty-one tumours, diagnosed by ultrasound as unilocular cysts were all histologically benign, independently of size. For complex and solid tumors there was an increased risk of malignancy, and a statistically significant correlation between tumour size and risk of malignancy was found. Using the ultrasonographic criteria for malignancy a sensitivity of 97% and a specificity of 28% was found. The suggested criteria for ultrasonographic suspicion of malignancy were found usable in differentiating benign and malignant ovarian tumours. The risk of malignancy for unilocular cysts seems to be low, independently of size.