Our objective was to determine the accuracy of three dimensional (3D) ultrasonography in the differentiation of ovarian tumours, Women (n = 20) with ovarian tumours (13 benign and seven malignant) were first examined using two-dimensional (2D) ultrasonography and were then evaluated with three-dimensional (3D) ultrasonography, All patients underwent surgery within 7 days of 2D and 3D ultrasonographic evaluations, and a histopathological diagnosis was made. A 2D ultrasonographic diagnosis of malignancy was done using an established scoring system. Diagnostic criteria for ovarian malignancy by 3D ultrasonography were: irregular thick septa, irregular papillary projection, mostly solid tumour, high echogenicity, and irregular inner wall. Compared with 2D ultrasound, 3D ultrasound had a significantly higher specificity (P < 0.005) and accuracy (P < 0.01), and a significantly lower false positive rate (P < 0.005). Our results suggest that 3D ultrasonography might be a better means of differentiating between malignant and benign ovarian tumours, However, in view of the small number of ovarian tumours, these observations must be considered preliminary.