The aim of the present study is to correlate transrectal ultrasonic planimetric parameters of the prostate in relation to age and urinary symptoms as evaluated by the American Urological Association (AUA) symptom index score for benign prostatic hyperplasia (BPH). In 647 examinees on a mass screening program for prostatic diseases using transrectal sonography (TRS) in Japan, prostatic volume, transition zone volume, transition zone index (transition zone volume/prostatic volume) and presumed circle area ratio (PCAR) were determined using transrectal ultrasonic planimetry and compared with age and AUA symptom score. Increase in age, prostatic volume, transition zone volume and PCAR were significantly correlated with AUA symptom score. However, multiple regression analysis demonstrated that age and PCAR were the only significant independent determinant of symptom score. In particular, PCAR was the only significant determinants of symptom score in men with an intermediately enlarged prostate (20-30 ml in volume). The most significant difference in AUA symptom score was found between subgroups divided by PCAR with a cutoff point of 0.8. Among the planimetric parameters obtained by TRS, PCAR was the most powerful for evaluating BPH in terms of the severity of lower urinary tract symptoms.