We examined the changes in tumor Doppler flow signals as well as prostatic volume in 11 cases of prostatic cancer, before and after castration, by using power Doppler imaging, and compared the changes in vascular resistance (RI) and prostatic volume (PV) in 11 cases. RI, except for one case, and PV decreased after castration in all cases without exception. In both, the manner of decrease showed an exponential curve. However, the reduction time of RI fell in an extremely narrow range compared with that of PV. Tumor vascular flow was influenced by castration, resulting in a decrease of RI. The dynamic change of vascular flow occurred earlier than that of PV. Analysis of changes in tumor blood flow signals could offer valuable information on early therapeutic effects in patients with prostatic cancer; however, it might not be a useful parameter for the prediction of prognosis.