In many cases of prostate cancer, changes of prostate volume were measured veryfrequently after castration by means of transrectal ultrasound.For short-term observations, a factor called as “reduction time (tau [τ]),” which means the time required for the volume of “effective portion” (volume responded to castration) to be reduced to one-tenth of its original value, correlated very well with patients\'prognosis.For long-term observations for relapse, changes of prostate volume could be divided clearly into two types, namely, “metastatic progressive type” with metastasis without volume increase and “local progressive type” with remarkable volume increase without metastasis.“τ” in the “local progressive type” was far shorter than that in the “metastatic progressivetype.”