As treatments for early localized prostate cancer come under closer scrutiny, the fundamental problem of documenting the success of radiotherapy becomes more obvious. Currently, no satisfactory method exists to determine tumor viability after radiotherapy. Transrectal ultrasonography is particularly valuable for monitoring the response of prostate cancer to radiotherapy. Persistent cancer retains its hypoechoic appearance after definitive radiotherapy. Hypoechoic lesions greater than 5 mm in diameter found more than 12 months after radiotherapy should be suspected of representing persistent local disease. In our study, albeit in a selected group of patients undergoing salvage radical prostatectomy, 92 per cent of such findings were associated with what we interpreted as viable tumor by light microscopy. Ultrasound-guided biopsy should be considered in such circumstances. The persistence of hypoechoic lesions in more than 65 per cent of patients 12 to 36 months after radiotherapy also suggests that local treatment failure may be underestimated by digital rectal examination and random digitally guided biopsy. Serial measurement of the diameter of hypoechoic lesions may provide a valuable indicator of progress in an individual patient. Patients with enlarging foci of tumor within the prostate after radiotherapy might be selected for biopsy and further treatment. If such a policy is employed, it is likely that a higher incidence of persistent cancer will be found after radiotherapy than has previously been discovered by random digitally guided biopsy.