Serial changes in the prostatic bed as identified by transrectal sonography in 31 patients with prostatic carcinoma were correlated with systemic change in disease. Seventy-three sonograms were performed, allowing interval change to be evaluated in 42 instances. Sonography demonstrating regression correlated with clinical response in 89 percent (8/9) of studies and with disease progression in 91 percent (10/11). Stable disease was correctly evaluated by sonography in only 56 percent (10/18) of studies. Six of those 8 incorrectly identified as stable were in men with prostatic irradiation prior to inclusion in this study. If they are excluded, stability was accurately gauged in 83 percent (10/12). Stability of the prostate on serial sonography may not accurately reflect clinical status in men with prior prostatic irradiation. In 1 patient abscess was superimposed on and could not be differentiated from tumor. Nodules of increased, decreased, and mixed echogenicity showed the same response to treatment as the total prostate volume.