We have demonstrated that the dynamic study for breast lesions by magnetic resonance Imaging (MRI) can differentiate benign from malignant lesions objectively. The cases were 57 histopathologically appraised breast lesions, including 20 cases of breast cancer, 28 cases of mastopathy, 8 cases of fibroadenoma and 1 case of intraductal papillomatosis. We plotted time-signal intensity ratio curves and then determined 95% confidence intervals, plotting the signal intensity ratio for both breast cancer and mastopathy every 30 seconds during dynamic magnetic resonance mammography (MRM) after gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) administration, and further established cut-off points to differentiate between them. We then tried of estimate objectively the benign-malignant differentiation to breast lesions by confirming their signal intensity ratio to be more or less than the cut-off points. We advocate this procedure, and call it the “dynamic ratio method.”As a result, we found highly significant differences between breast cancer and mastopathy at 30 and 60 seconds after Gd-DTPA administration (P<0.0001 ). We also confirmed that the cutoff point for the dynamic ratio method was equivalent to 1.4 and 1.8 times the precontrast signal intensity value at 30 and 60 seconds after administration of Gd-DTPA respectively. By performing this dynamic ratio method preoperatively we can assess objectively not only the malignancy of breast lesions, but also neighboring infiltration, extending intraductal component, and lymph node metastasis. Furthermore, the dynamic ratio method provides detailed information for selecting the appropriate region for breast conserving surgery preoperatively, and can be expected to reduce unnecessary biopsies of benign cases. The dynamic ratio method had a sensitivity of 95.0%, a specificity of 81.1% and a positive predictive value of 73.1%. Also, for detecting invasive ductal carcinoma, the sensitivity of the dynamic ratio method was 100.0%.