PURPOSE: To determine whether the compression technique is a valuable additional method for differentiating between benign and malignant breast masses. MATERIALS AND METHODS: The ultrasonographic findings of 95 benign and 53 malignant masses, all pathologically proven, were prospectively analyzed with regard to five diagnostic criteria: shape (regular/irregular), retrotumoral acoustic phenomena (posterior enhancement/posterior attenuation), internal echo pattern (homogeneous/inhomogeneous), compression effect on shape (distortion/no change), and compression effect on internal echo pattern (more homogeneous/no change). RESULTS: The number of cases of benign and malignant masses, respectively, was as follows: regular / irregular shape: 84/11, 9/44; posterior acoustic enhancement/posterior attenuation: 82/13, 16/37; homogeneous/inhomogeneous internal echo pattern: 78/17, 14/39; distortion/no change in shpae: 76/19, 5/48; and more homogeneous/ no change in internal echo pattern: 71/24, 3/50. For all diagnostic criteria for the differentiation of benign and malignant masses, the differences were statistically significant (p<.05). CONCLUSION: Ultrasonography is helpful for differentiating between benign and malignant breast masses. The compression technique is a valuable additional diagnostic method.