PURPOSE: To retrospectively determine the relationship between apparent diffusion coefficients (ADCs) obtained with 3.0-T diffusion-weighted (DW) magnetic resonance (MR) imaging and Gleason grades in peripheral zone prostate cancer. MATERIALS AND METHODS: The requirement to obtain institutional review board approval was waived. Fifty-one patients with prostate cancer underwent MR imaging before prostatectomy, including DW MR imaging with b values of 0, 50, 500, and 800 sec/mm(2). In prostatectomy specimens, separate slice-by-slice determinations of Gleason grade groups were performed according to primary, secondary, and tertiary Gleason grades. In addition, tumors were classified into qualitative grade groups (low-, intermediate-, or high-grade tumors). ADC maps were aligned to step-sections and regions of interest annotated for each tumor slice. The median ADC of tumors was related to qualitative grade groups with linear mixed-model regression analysis. The accuracy of the median ADC in the most aggressive tumor component in the differentiation of low- from combined intermediate- and high-grade tumors was summarized by using the area under the receiver operating characteristic (ROC) curve (A(z)). RESULTS: In 51 prostatectomy specimens, 62 different tumors and 251 step-section tumor lesions were identified. The median ADC in the tumors showed a negative relationship with Gleason grade group, and differences among the three qualitative grade groups were statistically significant (P < .001). Overall, with an increase of one qualitative grade group, the median ADC (A-A?A 1/2 standard deviation) decreased 0.18 x 10(-3) mm(2)/sec A-A?A 1/2 0.02. Low-, intermediate-, and high-grade tumors had a median ADC of 1.30 x 10(-3) mm(2)/sec A-A?A 1/2 0.30, 1.07 x 10(-3) mm(2)/sec A-A?A 1/2 0.30, and 0.94 x 10(-3) mm(2)/sec A-A?A 1/2 0.30, respectively. ROC analysis showed a discriminatory performance of A(z) = 0.90 in discerning low-grade from combined intermediate- and high-grade lesions. CONCLUSION: ADCs at 3.0 T showed an inverse relationship to Gleason grades in peripheral zone prostate cancer. A high discriminatory performance was achieved in the differentiation of low-, intermediate-, and high-grade cancer.