Introduction: There are relatively few studies that compare the use of transrectal ultrasound (TRUS) to magnetic resonance imaging (MRI) to estimate the prostate volume. In this study, we compared the prostate volumes measured with MRI and TRUS with a surgical specimen volume. Patients and Methods: Seventy-three patients underwent TRUS examination of the prostate prior to radical prostatectomy. All specimens were weighed and measured when freshly excised. The corresponding volume measurements calculated using TRUS and MRI were compared retrospectively with the measured volumes of freshly excised prostate. Results: The volume measured with TRUS and MRI was linearly related to the radical prostatectomy volume. The estimated increase in the prostate volumes measured with TRUS and MRI per specimen volume was 0.9508 and 0.9331 by regression analysis, respectively. If the prostate volumes were <35 cm3, the prostate volumes measured with MRI overestimated the specimen volumes. If the prostate volumes were >35 cm3, the prostate volumes measured with MRI underestimated the specimen volumes. The classic ellipsoid formula was adequate for determining the prostate volume. Conclusions: In this study, MRI and TRUS gave different volumes. MRI is more accurate than TRUS for determining the prostate volume. However, because TRUS is inexpensive, noninvasive, and almost as accurate as MRI, it should be the preferred method for measuring the prostate volume.