Prostatic volume in patients with prostatic cancer diminished remarkably after castration according to measurements by transrectal sonography. These kinetic changes in prostatic volume were analyzed and a formula was applied to the regression curve. The reduction time ( 7 ) in the formula, which is a constant representing the time interval required for the effective portion of the prostate to be reduced to one tenth, was thought to be a good index to predict the prognosis of the patients.