I-125 permanently implanted sealed-source radiotherapy is one of the treatments for early localized prostate carcinoma. In order to choose treatment parameters such as the number of implanted sources, accurately determining the volume of the prostate is vital for this type of therapy. We compared the usefulness of transrectal ultrasound (TRUS), computed tomography (CT), and magnetic resonance imaging (MRI) for preoperative stereometry of the prostate, and we further reviewed the utility of the MRI measurement. We performed prostate stereometry with TRUS, CT, and MRI in each of 45 patients who had provided informed consent. Taking TRUS stereometry as the standard, we computed the correlation of the MRI results with TRUS, and that of the CT results with TRUS, and found that MRI had a higher correlation.