Transrectal ultrasonography and ultrasonometrics were employed for follow-up in a total of 28 prostatic carcinoma patients subjected to external beam or interstitial radiotherapy. These two methods permit more accurate staging of prostatic carcinoma and have also proved to be valuable in the follow-up care of patients suffering from locoregional prostatic carcinoma. Of the 20 patients subjected to external beam radiotherapy four patients initially did not show capsular infiltration, 2B2, 2A2, whereas 16 patients presented with infiltration of the capsule and seminal vesicles. After external beam radiotherapy the ultrasonomorphologic findings of four patients revealed a sharply demarcated capsule and unremarkable seminal vesicles, which indicated tumour regression. Of five patients with infiltration of the pelvic floor and/or seminal vesicles, three showed definite tumour regression, whereas the ultrasonograms of the other two patients demonstrated tumour progression despite radiotherapy. In eight patients the greatest reduction in tumour volume was found one year after interstitial radiotherapy. Only one patient, initially presenting with slight infiltration of the capsule, was shown to have infiltration of the capsule and seminal vesicles after interstitial radiotherapy. At follow-up, evaluation of the echo patterns in these patients was inaccurate on account of the dense echoes reflected by the seeds implanted.