The purpose of this study was to investigate the diagnostic value of the apparent diffusion coefficient (ADC), measured by diffusion-weighted magnetic resonance imaging (MRI), for the diagnosis of breast lesions presenting as mass and non-mass-like enhancement (NMLE). The breast MRI studies of 174 patients were reviewed retrospectively. A total of 188 histologically confirmed lesions were analyzed and classified into 127 mass enhancement (86 malignant and 41 benign) and 61 NMLE (42 malignant and 19 benign). The ADC values were measured using a spin-echo echo-planner-imaging (SE-EPI) sequence with b=1,000 s/mm(2). Diagnostic performance was evaluated using receiver operating characteristic (ROC) analysis. The mean ADC was 0.99 ± 0.22 × 10(-3)mm(2)/s for invasive cancer, 1.23 ± 0.33 × 10(-3)mm(2)/s for ductal carcinoma in situ (DCIS), and 1.52 ± 0.35 × 10(-3)mm(2)/s for benign adenosis. The mean ADC of all NMLE lesions was 1.44 ± 0.41 × 10(-3)mm(2)/s, which is higher than the mean ADC of all mass lesions, 1.12 ± 0.33 × 10(-3)mm(2)/s. In the ROC analysis, the optimal cutoff ADC value for differentiating benign from malignant lesions was 1.05 × 10(-3)mm(2)/s for mass lesions and 1.35 × 10(-3)mm(2)/s for NMLE. In conclusion, ADC values can be used for the diagnosis of invasive and DCIS as well as benign tumors. The NMLE lesions tend to have higher ADC values than mass lesions; therefore, the morphological appearance of a lesion needs to be considered when using the ADC value for diagnosis.