PURPOSE: The purpose of this study was to correlate the tissue stiffness measured using ultrasound elastography with directly measured tissue stiffness. MATERIALS AND METHODS: A phantom model consisting of five materials with different levels of stiffness was constructed. Forty one patients who were scheduled to undergo a vacuum-assisted biopsy for breast lesions were prospectively recruited for this study. Elasticity (E) score and strain index obtained with strain imaging were correlated with the directly measured stiffness value of phantom and core specimen. RESULTS: The E score and strain index of each phantom material showed a high level of correlation with the measured stiffness (E score: r = 0.985, P = 0.002; strain index: r = 0.964, P = 0.008). The E score and strain index of the breast lesions showed a low level of correlation with the reference standard (E score: r = 0.261, P = 0.100, strain index: r = 0.397, P = 0.014). The mean value of the E score was significantly greater in malignant (3.6) than in benign lesions (2.4) (P = 0.004). CONCLUSION: Stiffness of the phantom materials assessed by US strain imaging was well correlated with the reference standard of direct measurement. The E score can be used as a non-invasive diagnostic tool in differentiation of benign and malignant breast lesions.