To determine the prognostic significance of a routine needle biopsy of the prostate performed six to thirty-six months after the completion of definitive radiotherapy, biopsy results were analyzed in 146 patients who had no evidence of disease at the time of biopsy and who received no other therapy before proved recurrence of the tumor. Patients were followed up a mean of 3.9 years after radioactive gold seed implantation and external beam irradiation. The total dose was 8,000 rad. Among 146 patients, 56 (38%) had one or more positive biopsy results within this time interval. The positive biopsy rate correlated with the clinical stage ranging from 17 per cent in Stage B1N to 59 per cent in Stage C1. The risk of developing local recurrence or distant metastases at any given time after irradiation therapy was markedly greater in those patients with a positive biopsy result (p less than 0.0005). Prostatic biopsy is an accurate means of measuring the success of radiotherapy. A positive postirradiation biopsy result carries grave prognostic implications for the patient and indicates that the treatment has failed.