Purpose: To compare 3D ultrasound (3D-US) with 2D ultrasound (2D-US) in terms of their usefulness and limitations in the diagnosis of breast masses. Materials and Methods: We obtained 2D and 3D US images of 37 breast lesions present in 20 cases of fibroadenoma, nine of cancer, and eight of fibrocystic disease proven in a total of 26 cases [fibroadenoma (n= 13), breast cancer (n= 9), fibrocystic disease (n=4)] by histologic examination, and by clinical evaluation and clinical evaluation with sonographic imaging in eleven. When comparing 3D and 2D-US images we had no prior information regarding detection rate according to the size of lesions, whether or not internal and boundary echo patterns could be interpreted, accurate differentiation between tumorous and non-tumorous lesions, or the accuracy with which benign and malignant tumors could be differentiated. Results: For lesions of 1 cm or less in diameter the detection rate of 3D-US was lower than that of 2D-US, but for lesions over 1 cm there was no difference between the two modalities. In fibroadenoma and breast cancer, 3D-US was more useful than 2D-US for the evaluation of both internal and boundary echo, but with fibrocystic disease and in the diagnosis of tumor/non-tumor, there was no significant difference. In breast cancer, however, 3D-US more accurately determined malignancy, and in fibroadenoma, because of the pseudospicule revealed by 3D-US, this modality was less exact in determining benignancy. Conclusion: In the evaluation of internal and boundary echo in breast mass diagnosis, 3D-US was more useful than its 2D counterpart. For lesions of 1 cm or less in diameter, however, the detection rate of 3D-US was low, and since in some benign cases a pseudospicule was apparent, the possibiliy of confusion with malignancy arose. For these reasons, the usefulness of 3D-US was limited.