Thirty patients who had transrectal ultrasonography before and after definitive radiotherapy were studied retrospectively to determine the effects of radiotherapy on the sonographic appearance of prostate cancer. Before therapy one or more discrete hypoechoic areas characteristic of cancer were present in 29 (97%) of the patients. In 10 patients (34%) the hypoechoic areas disappeared six to twenty-seven months (mean 11.4) after radiotherapy, but in 3 of these the hypoechoic lesion subsequently reappeared. Six months after radiotherapy a hypoechoic lesion could still be seen in the original area in 79 percent of 19 patients studied. Sonography showed persistent lesions in 65 percent of 17 patients at twelve months, in 79 percent of 14 patients at twenty-four months, and in 75 percent of 8 patients at thirty-six months. In 9 of the 29 patients (31%), there was a measurable increase in the size of the lesion, but overall, the size (maximum diameter) of the hypoechoic lesion had decreased by a mean of 41 percent when evaluated twelve months after radiotherapy. Previous studies from our laboratory have shown that persistent prostate cancer after definitive radiotherapy retains its hypoechoic appearance, and this study indicates that these characteristic hypoechoic lesions can be monitored by transrectal ultrasound.