PURPOSE: To evaluate the correlation between apparent diffusion coefficient ( ADC apparent diffusion coefficient ) values and the Ki-67 labeling index for luminal-type (estrogen receptor-positive) breast cancer not otherwise specified ( NOS not otherwise specified ) diagnosed by means of biopsy. MATERIALS AND METHODS: The institutional review board approved this retrospective study, and the requirement for informed consent was waived. Between December 2009 and December 2012, 86 patients with 86 lesions with luminal-type invasive breast cancer NOS not otherwise specified underwent magnetic resonance imaging, including dynamic contrast material-enhanced imaging and diffusion-weighted imaging with b values of 0 and 1000 sec/mm(2). Conventional measurement of the minimum and mean ADC apparent diffusion coefficient s by placing regions of interest and histogram analysis of pixel-based ADC apparent diffusion coefficient data of the entire tumor were performed by two observers independently and correlated with the Ki-67 labeling index of surgical specimens. RESULTS: For the interobserver reliability, interclass correlation coefficients for all parameters with the exception of the minimum ADC apparent diffusion coefficient exceeded 0.8, indicating almost perfect agreement. The minimum ADC apparent diffusion coefficient and mean ADC apparent diffusion coefficient and the 25th, 50th, and 75th percentiles of the histograms showed negative correlations with the Ki-67 labeling index (r = -0.49, -0.55, -0.54, -0.53, and -0.48, respectively). Receiver operating characteristic curve analysis for the differential diagnosis between the high-proliferation (Ki-67 ≥ 14; n = 44) and low-proliferation (Ki-67 < 14; n = 42) groups revealed that the most effective threshold for the mean ADC apparent diffusion coefficient was lower than 1097 × 10(-6) mm(2)/sec, with sensitivity and specificity of 82% and 71%, respectively. The area under the receiver operating characteristic curve (AUC) was 0.81 for the mean ADC apparent diffusion coefficient . There were no significant differences in the AUC among the parameters. CONCLUSION: Considering convenience for routine practice, the authors suggest that the mean ADC apparent diffusion coefficient of the conventional method would be practical to use for estimating the Ki-67 labeling index.