In a multicenter randomized trial of treatment for benign prostatic hyperplasia (BPH), three methods for determining prostate size were used. Data on sequential measurements of prostate volume in patients taking a placebo were used to determine the variability in the techniques. Suprapubic ultrasound scanning with measurements in three dimensions demonstrated a coefficient of variation of 30 %. Prostate volume determined by transrectal ultrasound and three measurements had in a variability of 12%. Step-section planimetry offered the lowest variability, 5%, and should be the method of choice in determining prostate size in clinical studies of BPH.