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Objective: To establish which method of determining prostatic volume (transrectal ultrasound [TRUS] or magnetic resonance imaging [MRI]) and which calculation formula give the most exact and least variable results; to determine the size and the source of the variability: and to establish which method is the more sensitive to drug-induced changes in prostate volume., Patients and Methods: Prostatic size was estimated by TRUS and MRI in 21 patients treated medically (either active treatment or placebo) for benign prostatic hyperplasia. Each patient was examined at baseline, and after 3 months and 6 months of treatment. Prostatic volume was calculated at every visit using different formulae proposed in the literature., Results: With some of these formulae, including the classical ellipsoid formula, there was a strong correlation (r > 0.8) between TRUS and MRI volume estimates. For others the correlation was much weaker, suggesting unreliability. MRI gave a significantly larger volume than TRUS because of larger values for the cephalocaudal and anteroposterior diameters. For patients on placebo the visit-to-visit variability of the prostate volume was 10-12% of the mean volume, whether calculated by TRUS or MRI. Part of this variability was apparently due to natural variation of prostate size., Conclusion: The classical ellipsoid formula is adequate for determining prostate volume. MRI and TRUS give different volumes. Visit-to-visit variability is similar for both methods and is partly due to real, natural variation. MRI is better able than TRUS to detect drug-induced changes in prostate volume.