Purpose: The purpose of this study was to determine the accuracy of manual semi-automated and volumetric measurements to assess prostate cancer volume on multiparametric magnetic resonance imaging (MP-MRI) using whole-mount histopathology for validation. Materials and methods: We evaluated 30 consecutive men (median age, 65.7 years; interquartile range [IQR], 61.5-70.9 years) with a median prostatic specific antigen of 8.5ng/dL (IQR, 5.5-10.5ng/dL), who underwent MP-MRI before radical prostatectomy. Index tumor volume was determined prospectively and independently on the basis of MRI and whole-mount section volumetric assessment using the maximum histologic diameter (MHD) and the histologic volume (HV). The MRI index tumor volume was determined by two independent radiologists using a single measurement of the maximum tumor dimension (MTD), a simplified MR ellipsoid volume (MREV) calculation and a MR region of interest volume (MROV) segmentation displayed by a commercially available OsiriX®. MTD was compared to MHD, whereas MREV and MROV were compared to HV. Results: Thirty index lesions (median HV, 1.514 cm3; IQR, 0.05-3.780 cm3) were analyzed. The MREV, MROV and HD were significantly correlated with each other (r>0.5). Inter-observer agreement for measurements was good for each method (r>0.780). The MTD was the best predictor of maximum histologic diameter (rCombining double low line0.980 and 0.791) and had an excellent inter-variability correlation (P<0.0001). Conclusion: Prostate cancer histologic volume can be assessed using MREV or MROV with a good accuracy and low inter-observer variability. MTD has the lowest inter-observer variability and provides best degrees of correlation with MHD. MTD should be used on MRI for selecting and following patients for active surveillance and staging before focal treatment of prostate cancer.