Prostate cancer (PCa) incidence rates have risen dramatically over the last few years with a particularly marked increase evident in the under 65 age group¹. A noninvasive method to predict aggressiveness of PCa would be desirable because it would help anticipate tumor recurrence and improve tumor management and the treatment outcomes. The Ki-67 protein is a marker of tumor proliferation, and apparent diffusion coefficient (ADC) and T2 relaxation time are related to tumor cellularity². The purpose of this study was to assess the relationship between MRI derived parameters (ADC and T2 value) and Ki-67 labeling index (LI) as determined from whole mounted radical prostatectomy specimens in peripheral zone prostate cancer.The local Institutional Review Board (IRB) approved this retrospective study and waived the requirement of informed consent. Between April 2013 and October 2015, fifty-two patients (mean age: 61.4 years, range: 42-76 years) who underwent preoperative MRI and radical prostatectomy were included in our study. Diffusion and T2 imaging were performed on a 3.0 T MR scanner to enable subsequent ADC and T2 calculation. A radiologist and pathologist located the dominant tumor on the MR images based on histopathologic correlation. Ki-67 LI was then determined using an immunohistochemical staining technique. The relationship between MR and histopathologic parameter was assessed using scatter-plots and the Pearson correlation coefficient. The mean tumor diameter was 15.2 mm (range, 5-28 mm). Of the 52 peripheral zone prostate tumors, four (7.7%) had a Gleason score of 6, 33 (63%) had a Gleason score of 7, and 15 (28.8%) had a Gleason score of greater than 7. ADC, T2 and Ki-67 LI in peripheral zone prostate cancer were (1.43±0.19)×10^-3mm²/s, (99±20) milliseconds, and 33.8%±5.3%, respectively. A significant negative correlation with Ki-67 LI was noted for both ADC (r= -0.696, P<0.0001) and T2 (r= -0.512, P<0.001). Trends for increased Ki-67 LI, decreased ADC, and decreased T2 with increasing Gleason score were also noted. A correlation between proliferation activity, PCa T2 relaxation time, and ADC values has been investigated. Increases in cellular density and disruption of ductal architecture in PCa can result in decreased ADC values. The Ki-67 protein functions as a nuclear antigen that is only expressed in proliferating cells. Noninvasive determination of parameters reflecting cell density may be important as some clinical studies have noted increased metastatic activity in highly cellular tumors suggesting that cellularity may be indicative of tumor aggressiveness. ADC and to a lesser extent T2 are good indicators of Ki-67 labeling index. Because of the potential link with Gleason score, multiparametric MRI may have a prognostic role with regard to potential metastatic activity and tumor aggressiveness. These results warrant further investigation on the potential of multiparametric MRI to facilitate noninvasive assessment of prostate cancer aggressiveness and proliferative activity.