The maximum horizontal area (MHA) of the seminal vesicles was measured in 20 prostatic cancer patients by means of transrectal sonography (TRS) after LHRH analog treatment. MHA of the seminal vesicles changed in parallel to the serum testosterone (T) level and decreased by 11-62%, compared with the baseline after LHRH analog treatment for 4 months. Two different patterns were observed for the change of MHA. In 9 cases, MHA of the seminal vesicles increased transiently and then decreased gradually, while in the other 11 cases, MHA decreased continuously from the beginning with no significant increase. Thus, the direct and dynamic response of the seminal vesicles to the change of serum T level was clarified.