In 175 normal men aged 27-70 (median 40) years, the prostate was ultrasonically scanned by the transrectal route. Ultrasonic signs of adenoma were found in 26%. The earliest age at which signs of benign prostatic hyperplasia appeared was 27 years, and the incidence increased with age. Adenomatous prostates were significantly larger than nonadenomatous, with median volume 28.0 (14.5-62.1) ml versus 23.0 (11.3-39.1) ml (p less than 0.0005). The adenomatous prostates also had a larger median volume of the periurethral gland (4.1 ml vs. 3.0 ml; p less than 0.0005) and a larger peripheral zone (23.3 ml vs. 20.2 ml; p less than 0.005). The total prostatic volume increased statistically significantly with age and the growth rate was calculated to be 0.81 %/year +/- 0.25 (S.E.) corresponding to 0.2 ml/year. The growth rate of the periurethral zone was 1.5 %/year +/- 0.5 (S.E.). Volumetric correlation between periurethral gland and total prostatic volume depended on whether or not adenoma had developed. Prostatic volumes measured in vivo by ultrasound are slightly greater than values obtained at autopsy.