A filled bladder acts as an acoustic window for transabdominal ultrasound measurements of intravesical prostatic protrusion and volume. The aim of this study is to evaluate the effects of bladder volume on transabdominal ultrasound measurements of these parameters. Twenty-two patients undergoing transurethral resection of the prostate (TURP) were studied. Under general anesthesia just before TURP, a transrectal ultrasound measurement of prostate volume was obtained. The bladder was then filled in a stepwise manner with 100, 200, 300, 400 and 500 mL. At each volume, the intravesical prostatic protrusion and prostatic volume were measured transabdominally using ultrasound. There was an obvious trend of decreasing mean transabdominal intravesical prostatic protrusions with increasing bladder volume. The mean transabdominal intravesical prostatic protrusion at bladder volumes 100, 200, 300, 400 and 500 mL was 9.1, 8.8, 7.4, 5.8 and 4.6 mm, respectively. The bladder volume at which maximum prostatic protrusion occurred was between 100 and 200 mL. The mean transabdominal prostate volume at the five increasing bladder volumes was 50.6, 48.7, 49.2, 47.9 and 41.4 mL, and these were correlated to transrectal prostate volume, particularly when the bladder volume was less than 400 mL. Transabdominal ultrasound measurement of prostatic protrusion is dependent on bladder volume. Transabdominal ultrasound measurement of prostatic volume correlates well with the transrectal measurement of the same parameter when the bladder volume is less than 400 mL.