Tumor volume has prognostic value in numerous malignant neoplasms; however, the determination of tumor volume in prostatic adenocarcinoma remains problematic. We tested the hypothesis that the diameter of the largest focus of carcinoma in whole-mount prostate sections predicts the volume of adenocarcinoma in the entire prostate. We evaluated 184 radical prostatectomy specimens by whole-mount processing of the entire prostate. The maximum diameter of the largest focus of carcinoma was measured directly on glass slides. Tumor volume in the entire prostate was calculated by the grid method. The maximum tumor diameter ranged from 0.1 to 4.1 cm (median, 1.6 cm). The total tumor volume ranged from 0.1 to 12.5 cm3 (median, 1.6 cm3). There were significant correlations between maximum tumor diameter and tumor volume (Spearman correlation coefficient = 0.84; P < .0001), surgical margin status (P < .001), perineural invasion (P < .001), serum prostate-specific antigen level at diagnosis (P = .004), Gleason score (P = .004), and pathologic stage (P < .0001). Maximum tumor diameter is a predictor of tumor volume and might be useful for the assessment of tumor volume in routinely processed prostatectomy specimens.