Transrectal ultrasonography of the prostate was performed before radical prostatectomy in 22 patients with persistent or recurrent cancer after definitive radiotherapy. Serial transverse sonograms were compared with whole-mount step-sections of the prostatectomy specimens to evaluate the sonographic features of irradiated prostate cancer. To define more clearly the effects of radiation on malignant prostatic tissue, we assessed the degree of histological change induced by the irradiation (radiation effects) as none, mild, moderate or severe. A total of 121 areas of cancer greater than 4 mm in maximum diameter were identified on the histological sections. Of these, 105 (87%) showed no radiation effects or mild effects, and 72% (76/105) of these appeared hypoechoic on the corresponding sonogram. There were 16 foci with moderate or severe radiation effects and only 25% (4/16) appeared hypoechoic. The remaining 12 foci were isoechoic. Areas of cancer which show moderate or severe radiation effects tend to become isoechoic while large (greater than 4 mm) foci of cancer in the irradiated prostate usually show little radiation effect, and these foci typically appear hypoechoic.