Purpose : To evaluate the effectiveness and the most accurate element of the diagnostic criteria of the JapanSociety of Ultrasonics in medicine(JSUM, Nov.1989) for distinguishing between benign and malignant solid breastmasses on the US. Methods and Materials : We analyzed the ultrasonic findings of histopathologically proved 51fibroadenomas, 12 fibrocystic disease, and 39 breast cancers in relation to the diagnostic criteria of theJSUM(shape, border, boundary echo, internal echo, posterior echo, lateral echo, and depth/width ratio). Results :The number of cases in fibroadenoma, fibrocystic disease, and breast cancer corresponding to the diagnosticcriteria was in the shape(26/51, 5/12, 33/39), border(41/51, 9/12, 29/39), boundary echo(48/51, 12/12, 27/39),internal echo(43/51, 9/12, 24/39), posterior echo(32/51, 3/12, 21/39), and lateral shadowing(15/51, 1/12, 35/39).All diagnostic criterias show statistical significance for differentiation of benign/malignant breast mass on theUS(Chi-square test: p<0.05). The order of accuracy was boundary echo, internal echo, and border. The mean ofdepth/width ratio was $0.54pm0.15,$ $0.52pm0.12,$ and $0.69pm0.21$ in fibroadenoma, fibrocystic disease, and breastcancer respectively and it has statistical significance for differentiation of benign/malignant breast mass on theUS(ANOVA test: p=0.0002). Conclusion : The diagnostic criteria of JSUM is effective for differentiation ofbenign/malignant breast solid masses on the US and has accuracy in the order of boundary echo, internal echo, andborder. Depth/width ratio also has statistical significance.