PURPOSE: We wanted to evaluate the ability of the apparent diffusion coefficient (ADC) values to differentiate between benign and malignant breast lesions and the normal breast parenchyma. MATERIALS AND METHODS: We used breast MRI, including DWI, to obtain images of 167 breast lesions (18 benign lesions and 149 malignant lesions) of 152 women (mean age: 48.6 years, range: 24-80 years). The mean ADC values of the malignant lesions were compared to those of the benign lesions and the normal parenchyma. We compared the ADC values of IDC, DCIS and other types of breast cancer and we also compared the ADC values with the nuclear grade of IDC. RESULTS: The mean ADC values of the malignant lesions were lower than those of the benign lesions and the normal parenchyma (p<0.001, respectively). The mean ADC value of IDC was lower than those of DCIS and other breast cancers (p<0.001, respectively). The mean ADC value of mucinous carcinoma among the other breast cancer was characteristic high compared with that of the normal parenchyma. There was no significant differentiation of the ADC values between the nuclear grades of IDC (p<0.828). The ADC threshold value of 0.98 x 10(-3) mm2/s for discriminating between malignant and benign lesion showed a specificity of 53% and a sensitivity of 100%, and the ADC threshold value of 1.33 x 10(-3) mm2/s showed a specificity of 93% and a sensitivity of 94% for discriminating between malignant and benign lesion. CONCLUSION: The ADC value is significantly different between the different pathological results of breast lesions.