We evaluated six alternative methods of prostate volume determination by transrectal ultrasound, three based on planimetry and three based on measurement of prostate diameters. Prostate volume measurements were made on an average of 6.5 occasions over a 3 y period on 41 patients with benign prostatic hyperplasia, using standard techniques. We defined the average of multiple planimetries as the prostate reference volume. Agreement with the reference volume and reproducibility at repeat testing was in the same range for single planimetry and volume determinations based on the formulas height (H) x width (W) x length (L) x pi/6 and W x W x H x pi/6, but was poorer using the formula W x W x W x pi/6. Using the average result of two successive planimetry measurements increased the reproducibility of planimetry, being statistically significantly better than for one single planimetry (P=0.024) or for the formula W x W x H x pi/6 (P=0.048). Our study suggests that the simple formula based methods of prostate volume determination provide results that are only marginally inferior to one single planimetry, but results are improved by performing two planimetry measurements.